I have to correct myself as I was wrong in my previous post. After some more poking I can confirm that indeed the recommended display size (as reported by IVRSystem::GetRecommendedRenderTargetSize) is different from what SteamVR “Settings” panel shows.
I did a new tests with ED, now also using Nvidia Nsight Graphics (which I found to be an interesting tool to inspect and record a whole lot of info about the frame rendering).
Here are the results for ED:
SteamVR SS override: 200%
ED HMD Quality: 1.0
PiTool Rendering Quality: 1.0
SteamVR Settings: 5448 x 4654
ED in game: IVRSystem::GetRecommendedRenderTargetSize() → 4096 x 3499
ED in game render target: 4096 x 3499
SteamVR SS override: 100%
ED HMD Quality: 1.5
PiTool Rendering Quality: 1.0
SteamVR Settings reports: 3852 x 3291
ED in game: IVRSystem::GetRecommendedRenderTargetSize() → 3852 x 3291
ED in game render target: 5778 x 4936
I also verified the disparity by running (again!) a simple app, which initializes OpenVR as VRApplication_Background and then calls IVRSystem::GetRecommendedRenderTargetSize. It reported 4096 x 3499 even when SteamVR settings override was at 150% and the UI showed 4718 x 4031.
So it seems that the limit is imposed by SteamVR/OpenVR. But when the app decides to use larger target (as ED did when pushed by HMD quality to 1.5) it seems to work ok. I am still however unsure where the final pre-warp happens as I did not find any (significant) reference to IVRCompositor in the frame rendering trace.