I’ll run the test, but what I think he is saying is that, were you to climb and expect the need to lean the engine, and leaned it accomplishing nothing, and leaned it accomplishing nothing, you will eventually succeed in leaning it to starvation.
In other words, leaning the engine now appears to be unecessary. Doing so anyway will accomplish no change in engine performance except to shut it down when the mixture control has been pulled out fully, or nearly so.
Let me give the G1000 a try.
FYI, the report that requested those changes for turbocharged aircraft in order to avoid leaning them as IRL:
I’m running the test now, but it occurs to me to ask…
How does a fuel injected engine behave as it climbs out?
Both C-172s I’m flying here have a fuel pump rather than a primer, which I’m assuming is an indication of fuel injection?
Does that change this discussion? Because the Cessnas I flew were O.L.D. So, that would put this even further out of my wheelhouse.
But it might explain why the OP is getting unexpected results.
Edit:
The results look pretty similar:
Standard Day
2330RPM Static.
2550RPM on 80KIAS climbout.
Down to about 2520RPM at 2700ft MSL
Only drops to 2470RPM by 6700ft.
To my knowledge the fuel pump is just used for takeoff and landing phases and don’t act as an injector so it should not impact the RPMs.
It’s difficult to say when you start asking Captain Google, but the AOPA site says that the C-172 (late model I assume) has the Lycoming IO-360-L2A, 180 hp engine.
Wiki indicates that the “IO-360” indicates the fuel-injected series…
All usual disclaimers pertaining to the Interwebz apply of course! 
For sure someone will confirm if G1000 & Steam share the same engine or not and if injected/turbocharged (I think it is different) or not.
In terms of requirement to lean there is no difference between a carbureted engine and a non-turbocharged fuel injected one: both experience diminishing manifold pressure as you climb resulting in reduced air flow causing an excessively rich mixture.
Note that the comments quote in @DementedCorn327 post refers to TURBOCHARGED engines: that is the difference. With a turbocharged engine, the turbocharger maintains the set manifold pressure up to the critical altitude. In the case of the T210 that I used to fly, that was at like 20,000ft at 75% power. Seeing as I never flew that high, power management was a real treat: after takeoff you set your desired climb power (MP & RPM) and forget about it until you reach your cruising altitude.
The first 172 to have fuel injection was the C172R in 1996. They changed the engine from the former O-320 series engines to a derated IO-360, with the I denoting fuel injection. This was carried over to the 1998 model year C172S, which I believe is the aircraft modeled in MSFS for both the steam and G1000 versions.
So I believe both steam and G1000 models in the sim are fuel injected but emphatically NOT turbocharged! FWIW a turbocharged engine would have had a “T” as the first digit in the engine model number
It occurred to me to try out something else. I looked for the most basic stock aircraft I could find.
It was more difficult than I thought!
But I think that the old school C-152 fits the bill. It has a fixed pitch prop, a primer, and perhaps most tellingly, Carb Heat.
So, I think this is a true, no frills carbureted engined aircraft.
Same start parameters.
Again, just shy of 2400RPM on static runup.
Increases to a little over 2500RPM at 70KIAS climbout.
Perhaps only 30RPM drop passing 3600ft MSL.
And maybe 2470RPM or so at 7000ft.
Just out of curiosity I tried to climb the 152 as high as it would let me at Full Rich. As you can see, still (barely) climbing at 9000ft. RPM has dropped off a little more to 2380RPM.
Now that the RPM had finally dropped appreciably, I inched out the Mixture to see if I could recover it.
I could.
With only about this much reduction, which isn’t very much.
So, I don’t know. In what I’m assuming are fuel injected (and non-turbocharged) engines, the RPM hardly drops up to altitudes of at least 7000ft.
In what are almost assuredly non fuel injected engines, the behavior appears the same.
Automixture is OFF. And reducing the mixture does restore RPM…once the RPM has dropped enough to notice. I assume that this means that the mixture control is operating.
So, over to someone who has flown more GA than I have to draw any conclusions.
But I do think that it has changed from what I recall it being before. 
Great stuff and thank you for it!
So, taken in whole (the 172 and now the 152 posted after your comment), does this seem like accurate behavior for these aircraft?
It feels less necessary to lean the mixture (for engine performance at least) now than previously. That feels incorrect. But it might just be an old bad simism that has only now gotten addressed.
I’m overwhelmingly a turbine guy via the mil, so this is pretty outside my scope. 
Remember the over-aggressive power loss I mentioned earlier? That was a bad simism. You’d have to lean it way back to something like 30% to make it have enough power to climb at stupidly low altitudes. If they finally fixed it, I am here for it.
You should only gain maybe 5% power by leaning, once you’re well into the climb. Leaning with the old system would sometimes make it a several hundred RPM increase, when in reality it should be several dozen.
In the real Archer, I don’t lean the mixture in climb (or takeoff) unless the DA is really high.
Glad to see it’s behaving better.
Just did a VERY quick flip in the steam C172. Took off and climbed to 10,00ft:
In real life the drop in RPM is accompanied by a VERY noticeable roughness in running that you FEEL. This they obviously cannot replicate int he sim.
Overall: while perhaps not perfect, this is modeled realistically enough to satisfy me.
For piston engines, the power falloff on the rich end is very shallow. Mostly you’re just sending excess fuel out the exhaust unnecessarily, but the power doesn’t significantly decrease until you’re brutally rich, at which point the power decrease curve steepens quite a bit.
Also note that RPM is not linearly proportional to delivered engine power, so is not necessarily a reliable indicator of the percent performance drop of the engine.
So far the posted tests look to be quite reasonable behavior.
Just one added thought here: I think one should be cautious in deciding what a “really high” airport is though. With the Turbo C210 I owned: no worries: always full rich, regardless! Many years ago now, but with the (normally aspirated) Bonanza I subsequently owned recall favouring testing leaning during the runup at anything over 2,500 ft just to be safe. DEFINITELY at 5,000ft, but then not many of us normally/frequently fly out of airports that high in real life I suppose.
Wow, so the fact that the mixture is finally behaving realistically after what feels like 30+ years of oversensitive behavior is pretty cool really.
It’s funny though how we become accustomed as sim flyers to bad sim behavior.
I absolutely recall having to unlearn a few things when I took my first 152 lessons in the '80s.
Hmmm, maybe that was to be expected…
Well, in reference to the OP’s question, and I never thought I’d hear myself say this with a straight face, but apparently…
“It’s not a bug. It’s a feature!” 
You do whatever the POH and/or instruction you’ve received call for. For a Lycoming IO-360, a lot of the time it’s limited to not leaning anytime you’re able to develop a certain percentage of power. That tends to fall off to “safe” levels in which you can’t exceed that limitation anytime you’re above 5,000’ DA. Taking off at 7,000 DA into rising terrain, I definitely lean it out a bit while doing a static runup.
Cessna 172S G1000 POH. page 4-30, before takeoff
“Prior to takeoff from fields above 3000 feet pressure altitude, the mixture should be leaned to give maximum RPM at full throttle, with the airplane not moving.”
This in addition to page 4-26 under “Engine starting”:
LEANING FOR GROUND OPERATIONS
For all ground operations, after starting the engine and when the engine is running smoothly:
For the Archer, it’s:
“Use of the mixture control in cruising flight reduces fuel consumption significantly, especially at higher altitudes. The mixture should be leaned during cruising operation above 5000 ft altitude and at pilot’s discretion at lower altitudes when 75% power or less is being used. If any doubt exists as to the amount of power being used, the mixture should be in the full RICH position for all operations under 5000 feet. To lean the mixture, pull the mixture control aft.”
Nothing specific about short field takeoffs or climbs, except a blurb in section 4-15:
“Prior to takeoff, set the mixture to the full rich position (for high elevation fields, mixture leaning could be necessary for smooth engine
operation).”
Regarding what airport elevation constitutes “high,” it’s literally as vague as it can be. This is why I go with 5000’ DA, which was taught back in my mountain checkout.
That’s fairly consistent with the Cessna 172P POH that I have too.
Worth noting for anyone who’s interested that % power isn’t linearly proportional to RPM though. Full throttle static RPM in the C172P is quoted as ~2400 while 2300 RPM in cruise corresponds to about 60% power.
2300 RPM is a good economy cruise setting in the game’s C172 and in real life. Carefully leaning the mixture while cruising at that RPM does result in a noticeable RPM peak.
I was flying the Barron a little while ago, going back knocking the dust off every aircraft in the hangar and lets say I wasn’t 100% attentive at all times. Mainly exploring t/o and landing dynamics and G1000 performance as I probably hadn’t flown it since it was updated. I set a climb to 12K and was a bit distracted and noticed later airspeed was dipping below 100KT and had to aggressively lean it and it roared back to life. It’s definitely modeled, including the buttkicker. Have to say it wasn’t unpleasant to fly and started spending more time with it. At the same time don’t care for the default Bonanza a bit, but had to lean it notably as well, both seemed right to me. I tend to fly most the N/A stuff under 8K’(usually ~1 hr or less flights) unless novelty antiques and the turbos above 10K for ‘going places’.
