For those still following this thread…
I got the 4090 installed and run the test scenario again to confirm my hypothesis. In short: it worked as expected. My FPS do not get higher than +/-50 in test scenario due to being CPU bound.
BUT
The 4090 is able to supersample the G2 to such extent that it actually feels like a new VR headset. I am not kidding.
First, let’s have a look at the test comparing 2080ti to 4090:
Previously, I ran OpenXR render scale at 100% and DLSS Balanced in the sim. Thus the sim was rendering at 1833x1793 and then the image got upscaled to 3160x3092. 40 FPS allowed me to be locked at 30 FPS motion reprojection in test scenario. The visual clarity was not perfect but fine for G2 and 2080ti.
Testing the new 4090, I settled for the moment on OpenXR render scale 200% and TAA 100%. This means that the sim renders at 4468x4372 and the image is then sent to the G2 with no further up / downscale. I use some sharpening. This setting allows me to run motion reprojection at 30 FPS in test scenario. Resorting to DLSS Balanced might push the motion reprojection to 45 FPS though at a cost of visible image degradation.
Oh boy, that clarity…
The clarity changes everything. In the sweetspot it feels like a new VR headset indeed. I know the supersampling is massive but translates to visual clarity. And that is what matters.
Now… give me better lenses for the G2 and I am in a flight sim heaven 
I can’t imagine how splendid it must be to run the 4090 with for instance the Varjo headset. @Troll, aren’t you planning a GPU upgrade? 