NO - I have a pair of 7850s and while the 2nd card sometimes shows a workload around 30%, I see no performance difference between windowed or full-screen.
I've tried forcing a Crossfire profile on the game through the Catalyst control panel. I think the AFR profile will enable a type of Crossfire, but all the textures go the hell - becoming transparent & constantly flickering. Warplanes is also having this issue, though that engine does seem to have a Crossfire profile already loaded into Catalyst. Nothing so far in Catalyst or in-game setting has helped with this problem.
My wife's R9 270 runs the game flawlessly at 1680x1050 on High settings at a constant 45FPS. (Just for reference performance)
I'm assuming we're gonna have to wait for a full Havok update. I'm still trying to get a clear reading on multi-core CPU support as well.