Same problem here I'm afraid.
On the 12.10 driver, on DX9 Low or DX11 High the performance is the same. I'm running a 6870x2 and if I disable crossfire performance improves dramatically. However playing with one GPU isnt an acceptable solution.
Using the Beta 12.11 driver and latest CAPs Far Cry 3 runs great with crossfire enabled but it causes a lot of my other games to crash so that too is not a solution for me.
I'm waiting for the next driver release and hoping that solves my problems. Anyone know when that might be?