First, thank you to those who responded. Unfortunately I find myself in the same situation again and I’d like to tell the story on the chance it might provide more information. Even before I read some of the suggestions, I contacted Gigabyte technical support an had a very unsatisfying conversation: the tech insisted that the problem was my 850 watt PSU and that since the 4090 is rated for 1000 watts I had to upgrade my PSU. In spite of my pointing out multiple inconsistencies in this formulation, it became clear that I would get no further assistance from Gigabyte so I installed a 1000 watt PSU but thereafter my PC failed to boot. So back to the shop where the tech himself called Gigabyte and even after rectifying the PSU problem, he got no help either.
He tells me it got a Kombuster score over 4000, but after only performing a minor Windows upgrade I run Time SPY and get another ridiculously low value of around 4000, a fraction of the best value the 4090 has produced of 25,000. None the less I then tried MSFS and had possibly useful results.
I first ran it in 2D, with no VR connected and used the Nvidia recommended settings. It looked pretty good! Pretty smooth and mostly Ultra settings with DLSS. Encouraged, I then plugged in my G2, and I IMMEDIATELY noticed degraded performance of my PC even while NOT running any game. In any case, sure enough MSFS in VR was absurdly bad, running in seconds per frame, and a quarter to half the image almost always blacked out.
At that point, I did do a reinstall of Windows, in a last desperate hope, but unfortunately that didn’t help either. I know this is long winded but I’m hoping it will suggest some possibility to someone because I see no possible routes to resolution at this point. Take it to a different shop? Support from Gigabyte? Give up on the 4090? All unwelcome alternatives.
P.S., On the remote chance this will help some “computer whisperer”, some additional strange behavior:
First, once something (whatever it is) goes wrong, it effects ALL VR games, not just MSFS.
It also affects 2D performance, at least in MSFS.
It affects booting. Frequently, almost inevitably the system will fail to boot. It will sound like it is, fans speed up then slow down, but no display.
Sometimes when this happens disconnecting my headset will enable the system to boot and sometimes I have to disconnect the headset AND my monitor (both connected via DP) and connect the monitor via an HDMI cable and it will then boot. That’s how I got here now after the latest round of such problems booting.