There seems to be a limit of 60fps when using 4K VSR which was very disappointing to me. It is a noticeable improvement in image quality/sharpness but it's not worth losing 144hz for, at least for most games I tried.
Been searching for info about this and there isn't much out there, DisplayPort itself is limited to 4K/60 or 1440/144, but VSR is doing the downscaling 'on the card' so that's not the limitation. In theory 4K VSR/144 should work but it's just not supported and doesn't look like it ever will be.