Crysis 3
Crysis 3 is a beautiful-looking game that continues to tax high-end systems. We've FRAPS'd a 30-second section near the start of the game, with lashing rain, numerous reflections and full DX11 pomp on show.
If Nvidia deems it economically and thermally possible to stick two GTX 780s on a single card, and it's a rather large if, we can't see how AMD will be able to respond anytime soon.
The gap really widens once the resolution is jacked-up.