It is most likely causing some extra processing because you’re going though an extra hop. However, the OpenXR and OpenVR are quite similar and any “bridge” is only a thin layer in the end (it is not like translating OpenGL into DirectX).
What matters the most in the end is the final render resolution as it is determined by SteamVR SS + per-app SS and shown in the per-app video settings popup panel in SteamVR. This determines the resolution FS2020 is rendering to if FS2020 Render Scaling is 100% (at least this is how it works with the Index).
You might want to read my [Update 08DEC2020 - Render Resolution (continued)] addendum here:
https://forums.flightsimulator.com/t/my-2070-super-vr-settings-and-suggestions-valve-index/321913/2
NB: you can see the render res and post process res using the FS2020 dev mode fps overlay in VR mode. It will be displaying this info on the monitor directly (just lift the headset to peek).