There's no comparison to be made efficiency-wise between Mantle and DX. One blows the other out of the water. The difference is not small, and it is not 5%, at least in multiplayer.
http://www.pcgameshardware.de/Battl...tlefield-4-Second-Assault-Benchmarks-1109970/
They did a follow-up to see what it took to get Nvidia cards to par with AMD cards (Windows 8.1 and a 4770k @ 4.6Ghz):
http://www.pcgameshardware.de/Battl...ield-4-Second-Assault-Geforce-Radeon-1110584/
As you can see, they're still behind, and still probably CPU limited-- you can see no change between the 670 and the 770, or between the 560Ti and the 570. The differences between these two pairs of GPUs are so great that I doubt the unpredictable nature of multiplayer testing will cover it up, and they probably ran through several times on each GPU anyway. On the AMD side, there is a clear progression in performance between the slowest and fastest GPU tested.
Even if their average matches up though,
averages can hide really poor behavior. There's probably a point where you get no returns and you're purely "GPU limited" but if it doesn't happen with Haswell @ 4.6GHz then it is a point that is mostly academic. And I'll assert, though I'm too lazy (and poor) to try and prove, that Mantle will have a much tighter spread of frame rate no matter what CPU you use.
My experience with Mantle is largely positive and reflects these results, on a 4770K both at stock and overclocked to 4.1 Ghz and a stock R9 290.
...5%
. Even if there were no graphs that prove otherwise, I would still laugh at that number. I could feel the difference straight away. And my 4770K... well, it wasn't cheap.
Sorry, I had to make this account because I was so shocked at those results.