Considering Jayz2Cents just recently did his GTX 1080 review and showed.. I forget what game, but it's in his youtube video, but the point is his test showed a DirectX-12 game using 9GB of video ram in his Titan X.. just at 1080p. And 4k would use nearly +4x of that. In fact in his test the AMD R9 Fury X was the lowest in the test most likely because of it's 4GB of ram vs the nvidia cards with 8GB and 12GB.
I'm looking forward to 24GB cards.
Guessing it was Tomb Raider? DX12 is completely broken in it.