Odd, switch over to 14.2 and AB will not keep a constant core oc. In heaven its going up and down on the core. With 13.12 I can keep a constant flat line 1200 core.
What do you mean "won't keep a constant core OC"? Is it throttling? The GPU will be working harder with Mantle. I've read where some O/C's aren't stable in Mantle and have to be reduced. Again, likely because the GPU is working harder and the OC isn't as stable as first thought.
As far as Mantle's Memory usage being higher. Remember the game's dev has the freedom (responsibility) to allocate how the card is going to use RAM. It's likely some tweaking needs to be done.