> From what I understand, it's more accurate to say that it reduces CPU use by about 50% (rather than improving performance)

According to this blog entry, you are mistaken, and it really can improve performanec 54% in a multi-GPU rig.

http://battlelog.battlefield.com/bf4/news/view/bf4-mantle-li...

There seem to be issues with FPS drop, though ('stutter').