I got my 12900k yesterday and wanted to share some first impressions regarding MSFS.
Previous system:
11900k @ 8x51, HTT=on
Asus Z590 Apex
2x16GB Gskill 3800-C14 @ 3866-16-16-16-36-2T Gear 1
RTX3090 Aorus Xtreme +50/+500
CPU cooling: Alphacool Eisbaer Extreme + 420mm radiator
Win10 latest updates
NVidia 496.49
100Mbit/s
New system:
12900k @ 54/54/54/53/53/53/52/52, TVB+2, HTT=on
Asus Strix Z690-A Gaming D4
2x16GB Gskill 3800-C14 @ 3600-16-16-16-36-2T Gear 1
Rest of the specs see above.
I’m using a Reverb G2 with WMR + OXR 1.07, Motion Repro Off. Nvidia Control Panel everything at default except Power Management = max. and Texture Filtering = Performance.
Benchmarking was performed using a CapFrameX and a pre-recorded 6min test flight over photogrammetric city, rivers+lakes and dense forests with a DA40 TDI in cockpit view at a fixed start time & date. During the flight, the headset was resting untouched on the table, view centered. Dev mode overlay was on, which usually gives a penalty of 1…2 FPS. Results are average over 3 runs. In-game settings are a mix of high+ultra plus a few medium settings, see below. Standard ist OXR=100%, TAA=100%, but I also compared 200/50.
Edit: I’m fully aware of the fact that I was running GPU limited pretty much all of the time, but that was on purpose because it will reflect what most of us are experiencing in VR. Still, the CPU may have an effect on minimum and low FPS.
As expected the gains over the 11900k are pretty small, though the increase in min FPS is very welcome. The CPU is pretty much maxed out, the MEM isn’t but that won’t make much of a difference. Things could change, however, if DDR5 matures and we see RAM kits with 6000+ and CL36 or less at a reasonable price. But that’s probably gonna take another half year or more and until then, at least for me DDR4 is the way to go. Enabling/disabling E-cores did not affect performance directly, but it is obvious that one is losing a bit of O/C headroom for P-cores when E-cores are active. So if you want to max out the P-cores and Ring, leave the E-cores off.
So bottom line: If you’re coming from a 11900k the upgrade is certainly not worth it if your aim is improving MSFS performance. I use the PC mainly for work and here the 12900k delivers. The +1…4 FPS in MSFS are a welcome side effect. If you’re still on an older gen CPU, you may consider the upgrade, but then again I would wait until decent DDR5 is available. I came from 10900k before the 11900k and compared with the 12900k I’d say I’ve gained something like 6…7 FPS - roughly, considering performance at equal settings but also differences due to SU5 & 6.
Happy flyin! 
VR Settings
TAA
TLOD 200
PRE-CACHING ULTRA
TERRAIN VECTOR DATA ULTRA
BUILDINGS HIGH
TREES ULTRA
GRASS MEDIUM
OLD 200
CLOUDS ULTRA
TEXTURE HIGH
ANI FILTERING 16X
TEX SUPERSAMPLING 4X4
TEX SYNTHESIS ULTRA
WATER WAVES HIGH
SHADOW MAPS 1536
TERRAIN SHADOWS 512
CONTACT SHADOWS HIGH
WINDSHIELD MEDIUM
AMBIENT OCCLUSION MEDIUM
CUBEMAP 64
RAYMARCHED REFL LOW
LIGHT SHAFTS HIGH
BLOOM ON
COCKPIT REFRESH HIGH