Great post and thanks for doing the hard work comparing the cards. I would argue that this is only relevant for this very simulator, since NVIDIA cards still have things AMD has not yet built in. DLSS is one feature missing from AMDs portfolio. Raytracing is anohter - both do not play a role in msfs 2020 yet. ( And you can argue about the value of raytracing in general I guess…)
Having that said, I recently invested in VR. I do have a AMD card and can confirm that the AMD drivers for this are less than optimal. I get constant “Graphic driver timeouts” which always lead to a CTD.
System:
- AMD Ryzen 7 3800X
- 32 GB RAM
- 1 TB NVME Flash drive for Simulator
- Sapphire RX 5700 XT
First I tried the Oculus Quest 2 and must say - hands off if you have an AMD card (no matter if 5x or 6x). The Oculus Link “Technology” is only poorly running on AMD cards. The reason for that is that your card needs to do 2 things in paralell:
- Render the game in VR resolution (which is high for the Quest 2 - good display!)
- Encode the rendered came content into a video stream to send to the Oculus Quest 2 HMD
The Encoder in AMD is very poor. It performce not nearly as good as the NVIDIA cards. Reason for that is simply that NVIDIA has its own business branch focussing on enterprise graphics (Quadro) and they invested a lot in hardware encoders. The 3x series (and even the 2x series) have this tech built in.
Now this is only relevent if you are usng a Quest 2 - but I can tell you - It is no fun with an AMD card (drivers and encoder performance).
I managed to get my hands on a HP Reverb G2 (which also is tricky to run on any AMD card - but it runs! - Yay!). This one runs sooo much smoother even though the headset requires even more GPU power (higher res). But I can tell you it runs at least twice as good as Oculus link. Now, I only get driver timeouts once a week or so… 
Now I would wonder how these two cards (the 6900 and the 3080) compare in the VR space - since it seems you cannot simply take results from “non VR” benchmarsk and transfer them.
I am mediocre happy with my AMD card, that is why I actually plan to switch to a 3080 once the prices become reasonable again (summer ;D?)…
Just my experience…