From openvr_fsr release notes:
“fixed foveated” optimization: FSR is now only applied to a configurable radius from the image centre, any pixels outside the radius are upscaled by cheaper bilinear filter. Even with a fairly small radius, it is often hard to tell the difference due to the current HMD lenses. Feel free to experiment with it and gain a few additional usecs for your frametime budget
As FSR is Open Source, couldn’t it be integrated in PiTools?