Just because such a device would (hopefully) have dual 8K screens, does NOT mean you need Quad SLI Titan V's per eye. You do not have to render every pixel, nor do you have to run Crysis 4 on it. Also, remember that the Vive and Oculus have very narrow FOVs, which should be improved. Doing so would stretch out the pixel density by about 2x horizontally, not vertically, so you'd need more horizontal pixels to get the same quality picture, and 2K/4K/8K terminology measure horizontally.
So, all of this adds up to dual 8K screens not being anywhere near as difficult to run as people here seem to assume. You could run respectable resolutions at reasonable graphics quality on one or two current high-tier gaming cards, then upscale the resolution to show on the 8K screens. It will still look fantastic (slight blurring, but no screen-door), much better than a lower resolution screen.
Obviously I'd hope that such a device could be used in both OSX or Windows to maximise usefulness, however if it ends up being a dedicated iOS style closed box, then you could probably get the required Metal-enhanced performance from the equivalent of 4 iPad Pros.