I currently have an i7 10700K with an RTX 2080Ti
If I changed the i7 to an i9 10900K (the max my mobo can take without having to upgrade mobo too) would I see much of an increase in FPS with the 2080Ti?
I plan to upgrade the 2080 to a 3080Ti, when funds permit. But should I put the money I have for the CPU upgrade into the GPU upgrade first, and wait to upgrade the CPU, and concentrate on upgrading the GPU first?
Nope - maybe +1 or 2.
Example
i7 12700K vs i9 13900K +10 FPS.
Neither. The NVidia 3060s are being heavily discounted at my local supplier, but STILL more expensive for worse performance than the Intel Arc 770LE. You should be able to do 4k with the Arc.
And FPS isn’t the be-all and end-all people make it out to be. Not in a flight sim. There’s a lot of fake 120fps and 240 fps panels out there using tricks like black frame insertion and interpolation to get a claimed equivalent to 240 fps out of 60 fps panels.
People still haven’t gotten in through their heads that if a pixel is the same value between “refreshes”, it simply doesn’t need to change values - there’s no need to “refresh” it, unlike the previous century’s CRT tubes. So only areas that change values need to be updated. Mostly moving edges or values when lighting changes (which doesn’t happen all over the screen even 30 times a second in many scenarios).
And of course, the higher you fly, the less rapidly the scenery changes, which is why FPS rates go up the higher you fly. Also, clouds don’t need as much GPU horsepower as the ground does, so clouds are relatively static compared to ground cover.
And if you’re using the interior view, the cabin mask that covers some of the outside scenery doesn’t need as much updating either.
So get yourself an Arc 770 LE. 16 gb of VRam, a 256 b it data bus, clock speed is within 7% of an NVidia 4090, and it can be overclocked 25% (not that you’ll need to).
It’s become a hot item, so there are some supply bottlenecks, but for the price !!! I can wait another month for a second one (but I am counting the days, for sure).
if you have the money for the 3080ti then get a 4000 series instead, you will get double the fps with DLSS 3.00 and Frame generation, makes for a real smooth ride 
Well it would be a 2nd hand 3080Ti… Probably around £700 or so… Id need to add about £1100 onto that to buy a new 4090… Just cant justify it at the moment… 
yeah they are crazy expensive, however all 4000’s use DLSS 3.0 and frame gen and will boast your fps over any other card in MSFS
the 4070ti are much cheaper, not sure how they perform against the 3080ti, but with DLSS 3.0 and frame gen I.m sure they would give you more fps and much smoother flight?
but I would check out some reviews of each card running msfs first
If your MB supports Gen 4 m.2 and PCI-e slots, going to the 11700 or 11900 from the 10700K (rather than the 10900) will double your bus speeds. Won’t help much for FS 2020 but your files will load faster. 
A 4070Ti is almost in every game 3% quicker (also in MSFS) than a 3080TI without DLLS3 mode active.
There are many, many benchmarks on the internet. With DLLS3 the frame almost double!
I had a 9700K and 2080 upgraded to 3080ti (whqd monitor). The frame rate has gone up but not much<5%. Recently I upgrade to a new motherboard with a 13700k. Now i have decent framerates with the 3080TI but still cpu maintreathed…So go for DLLS3 (4070TI)
And the 11700K and 11900K Cpu-s have 20% faster single core performance which helps.