3080 has 10Gb.
But I made progress: I just had a great first flight, finally! The problem turned out to be with the SU10 beta. I was trying and trying and I could not get a stable framerate: today I tried maybe 10 times with full reboots, but I always got the FPS pulsing and horrible stutters. Once FPS started out nice (31 FPS + ~58% CPU Overhead) but it quickly deteriorated into a total mess, as before. All other times FPS pulsing started right away. So I gave up and switched back to non-beta SU9.
And lo and behold, it’s totally smooth, no FPS pulsing! I used 100% TAA, OpenXR Toolkit FSR at 80% (also had good results for up to 87%!), sharpening 100%. So it’s similar quality as DLSS, only sharper (but more aliasing/shimmering than DLSS). FFR on quality/wide preset, ultra clouds, the rest medium/high. I had an amazingly smooth flight in Cessna 310 in New Zealand, and then I rebooted and loaded the ultimate test: PMDG 737 in KJFK, short full flight to KHVN with Scattered Clouds preset, live traffic and all players. And it was smooth most of the time! I did have occasional small stutters, possibly when new scenery was loaded etc. - normal stuff for MSFS (unfortunately). But otherwise it was locked at 30 FPS MR + 20-50% CPU overhead for about 80% of the time, occasionally lowering to 22.5% MR but there was no regular dipping below that and no pulsing.
So there is something fishy going on with SU10 - I got FPS pulsing with or without OpenXR Toolkit and just could not make it flyable, despite good potential of DLSS. That worries me a lot, because when SU10 will be released, it can really hit the fan… I opened the bug report here, please vote if you have a similar issue. FPS Pulsing (oscillating FPS) and heavy stuttering in VR - impossible to achieve stable VR with DLSS SU10, FPS deteriorates fast