Sooo... I just gave
https://github.com/fholger/openvr_fsr a shot with Elite (it replaces the game's copy of openvr_api.dll, and injects AMD's FSR between the game and SteamVR), trying a few different resolutions, upscaling ratios, and sharpening strengths...
I don't know... I can't say I, to my taste, discern getting any better image quality to frame rate ratio, than I do by simply letting the game render the same actual rendered resolution normally, without any subsequent "smart" upscaling.
The main problem should of course be the good old: "Sheet in, sheet out"... If the resolution is too low for the game to choose to generate a denser LOD, and then have the resolution to render it out with minimal aliasing or blur, we're upscaling a sparser one, and just "magnifying" its problems.
Mentions are made here and there in the game's config files, of LOD tables... I don't know whether one can try editing those, and see whether that could yield anything, instead of just brute-forcing supersampling, but have to imagine their levels already being optimised for a reasonably 1:1 relationship; Push it, and you are likely to suffer more aliasing; Relax it and: hello lowres...
IMHO; For a Valve Index (about 15ppd), the new terrain
needs an absolute minimum x1.5 "HMD Quality" (...or equivalent in e.g. SteamVR render resolution multiplier (EDIT: 225%))
plus x1.25 of the game's inbuilt supersampling; The former for detail and sharpness, the latter for "pre-swallowing" the worst aliasing - both contributing to a more "stable" view.
Below these numbers, the LODs just don't look right, and are too blurry, to boot.
Others may have different sensibilites.