mantle only worked on GCN 1.1, not older card or newer cards like Tonga. Then it was only available in a few games, people who played different games even with a GCN game had not benefit, so we are really talking like a couple of percent at the most.
Rubbish. the problems with mantle in BF4 were due to Dice's implementation. nothing to do with the API not working on GCN1 cards.
I had it running fine on my 7870 2GB. The problem was that dice had memory leak issues which pushed up the vram usage higher than need be. but if you altered your settings so it did not hit your cards memory limit. the game ran silky and had no issues.
It was only once it kept hitting your gpu memory limit that you had stutter and issues on map loads.
Also the main benefits were in min and average FPS which hardly any sites used in their reviews. And mantle benefited the most in multiplayer when the draw calls jump at random. compared to single player which most sites bench marked the game with. where the maps are carefully crafted to reduce draw calls.