Something weird is going on with those hyper benchmarks. Normally when you're VRAM limited your framerate tanks, but you shouldn't experience that the game outright refuses to run.
I suppose it is possible that the game has a built in VRAM check when you turn on hyper settings, but that means that it's effectively impossible to say exactly how much VRAM is necessary. You quite obviously don't need 8GB for 4K + hyper settings, seeing as the 6GB 980 Ti performs exactly as you would expect (i.e. 5% slower than 1070 and 25% slower than 1080).
Very true, the VRam "brick wall" should tank you to single digit fps. 6gb shouldn't be enough per nvidia, but 8gb seems exaggerated per logic.
Something weird going on?
Edit: unless it's uncompressed textures?
It ran, just had stability problems.
From the review, right under the chart that shows the 970 and Fury X at 0:
"The Fury X and GTX 970 refused to run with any stability we think that's a VRAM limitation and so the chart only shows the 390X, 1080, 980 Ti, and 1070."
If you really do run out of VRAM, the game will essentially be a slideshow. This happened to me when I tried maxing out Far Cry 4 with AA @ 4K using DSR, just to see what would happen. It was difficult to even get to the menu to quit...