I've tried pretty much all the VR videoplay (virtual desktop, simpleVRVideo player, Whirligig, DeoVR, etc..) and have the reverse experience. SD titles looks ok on monitor, but I can quickly tell in VR. 60fps helps with the motion, but the overall video is still soft (slightly blurry, darker) This goes for non-vr titles too (I do watch regular vids in VR Theater)
SD Jav titles looks ok on monitor, but when view in a virtual, movie theater sized screen, the low res is pretty noticeable compared to a FHD version.
And like you said, the huge downsize of FHD titles are their filesizes. Between FHD Jav & VR, I run maxing out my hdd each month. So I have to go thru and see which one to save (externally) and which one to delete