Results: Assassin's Creed IV, 2560x1440
At the highest detail settings with 2x MSAA and soft shadows enabled, the only AMD card that comes close to a 30 FPS minimum frame rate in Assassin's Creed IV is the Radeon R9 290X.
Asus' Mars 760, on the other hand, does very well, nipping at the GeForce GTX 690 and besting our overclocked GeForce GTX Titan. Presumably, that means it'd also beat a GeForce GTX 780 Ti).
While the Mars 760 does register a notable spike in frame time variance, it's both brief and low in amplitude. The rest of the cards perform admirably, including the slower Radeons.