Does mantle come to a grinding halt in some situations on bf4 eyefinity? Yes.
Particularly if i'm in a tank and I engage the aps and a tankbuster is strafing me, the close up explosions can dive bomb the game to single digits, whereas on dx it'll stay around 35-40fps.Its a pretty common situation while driving a tank around and currently mantle doesn't do a good job of providing a good experience at 5760x1080.
I get the same at 1920x1080. If 2 jets collide and explode both my GPU and CPU graphs spike and my FPS come down. I don't know if they go in to single figures though.