I can't have it enabled without Crossfire performance immediately going to 50%.

Its easy to check. Watch X-Fire scaling in game, and then toggle r_PostMSAA on/off. Different settings with r_PostMSAA and UseEdgeAA do nothing - if AA is on, then Crossfire gets half performance.

Anybody know how to force AA on this game?

EDIT: Forgot to say that this happens only when sys_spec_PostProcessing is at 4.