Finally, with the High preset enabled, and a Radeon R9 290X/GeForce GTX 780 Ti installed, the results start getting strange. Nvidia's high-end gaming card averages about 80 FPS on a Core i7-4770K, and is matched by the Radeon R9 290X using Mantle and an FX-8350. Then, we swap the R9 290X into our Core i7-4770K-based system and observe dismal results with Mantle turned on.
The numbers are repeatable, and we don't have a concrete explanation to put forth. But we're reminded that AMD considers Mantle to be a technology in its beta stage. Overall, though, Mantle continues to help performance trend higher than DirectX