Virtually none of the Canon zooms can resolve at this resolution at working apertures before diffraction kicks in anyways. The new 16-35 maybe, and almost certainly a number of the Super-teles, but who needs 50mp when shooting with a 200-400?
Nikon has this same issue with the D800/D810 and Sony the A7r. The 14-24 barely gets sharp enough in the corners for the D800E at f5.6, yet at f10-11 there is already quite noticeable diffraction. I won't bet on any of the E mount zooms having enough resolution for the A7r either. The 55 1.8, on the other hand, does make for a great combo beyond f2.8.