I have messed around a bit with this - some rambling ahead, forgive me. For reference, I'm using a Reverb G2. I got it working with the latest OpenXR-branch of OpenComposite and the latest OpenXR Toolkit. Pretty cool. I always felt that the WMR-SteamVR bridge was a severe bottleneck for my headset.
First major drawback for me, and maybe important if you want to try this out: EDHM (more precisely: The 3DMigoto wrapper dll) makes OpenComposite crash. So, back to orange for my testing...
First tests were promising. I did not do an extended playtest, but things seem smoother, less "jerking" and microstutters in the headset. But for a proper comparision setup to SteamVR, I'm a little confused. I'm not sure if I am comparing apples to apples or oranges.
First issue: Render resolution. There is a render scale slider in the OpenXR-Tools settings panel, and there is the render scale setting in opencomposite.ini. I'm guessing leaving the render scale option in OpenXR-Tools disabled enables some kind of auto-scale. I also figured out that that slider setting and the opencomposite.ini-setting multiply. I am guessing it's similar to the global and per-app percentage setting in SteamVR?
If that's all correct, I left the render scale slider at 100% and supersampleRatio at 1.0, which would be equivalent to a resulting 100% scale in SteamVR. Inside a coriolis (that's where I'm sitting right now) I get comparable performance to my SteamVR setup (where 70% is usually the max I can run without having a stutter orgy). So that's a win, I guess.
I'm not really sure, but things seem less... crisp? But that could be an effect of having to play with default color setup (no EDHM). With the EDHM I usually run a dark blue HUD setup with white text and little to no cockpit lights, the default orange always seems overblown in contrast. I don't know, guess I need to do a few proper comparison sessions.
Second thing that doesn't seem to work is the automatic motion reprojection. With SteamVR, not using MR wasn't an option, because all framerates that weren't 90 or 45 lead to major stuttering when I turned my head. So Inside a station I usually get 45 fps with MR.
With OpenComposite I either have to force it on in OpenXR-Tools and have it on all the time or have no MR at all. Two things I noticed: One, head motion without MR was still very smooth even at 55 to 60 fps (which is what I got in the station without upscaling), but the fps overlay of OpenXR Toolkit stuttered like hell. And two, with MR enabled I got locked 45 fps, but the framerate display sometimes dropped to 30 fps for a few seconds, although visually everything was fine. Don't know what to make of it.
My plan to compare OpenComposite with SteamVR on a fair basis: Set supersampleRatio to 0.7 (70%), lock MR for both and do a bounty hunting session. My expectation is that the experience in OpenComposite will be much smoother. If that's the case, I guess the next thing will be upping the resolution and play with upscaling.
In terms of upscaling: Does anyone have an opinion on NIS vs. FSR?