Tinkering needed, but can run gameplay at 60fps.
After applying the listed fix steps, played for about an hour so far - various songs on various difficulties. Lobby has severe visual issues, but from song selection onwards I had no further problems. While in a song and doing rhythm gameplay the visuals are fine,the tracking and movement timing seem as good as native. Depending on tweaks I was able to get either 60fps (with frame drops) or 54fps (solid), reprojected to 120 and 108 respectively. The performance seems to be CPU-limited, so reducing visual quality further doesn't seem to improve frame rate.
Properties > General > Launch Options: Set to "OPENSSL_ia32cap=:~0x20000000 %command%" (no quotes - required to fix a crash soon after startup) In-game settings > Motion Graphics: Set to "Off" (fixes frame drops on hitting notes) VR Video Settings > Refresh Rate Override: Turn on and set to either 120 (for 60fps with occasional drops) or 108 (for solid 54fps)
Lobby only: Severe flickering textures and poor frame rate.