From what I can tell crossfire performance in Crysis DX10 doesn't scale hardly one bit... I did some benchmarks and here is what I got with an "Ultra High [HD]" config file. Looks better than Very High and can use it on DX9 and DX10.
1680x1050
DX9
No AA:
Min: 28.17
Max: 66.28
Avg: 46.6
4x AA:*
Min: 20.24
Max: 48.64
Avg: 33.045
8x AA:
Min: 24.41
Max: 53.63
Avg: 40.225
16x AA:
Min: 24.66
Max: 53.85
Avg: 40.52
DX10
No AA:
Min: 18.27
Max: 45.94
Avg: 28.97
*Graphics corruption observed.
While playing the game, it's really strange... I have say 60FPS, then if I zoom in with either a scope (sucks because I like sniping! luckily though it doesn't affect the reflex sights so the game is still playable ) or the binops my FPS halves and I get mouse lag, even after i've zoomed out again. Only way to fix it is to quit the game and restart it! Also AA from the game menu has big FPS impact - unlike the benchmark which showed hardly any decrease. I can only conclude from these two things that crysis is royally fubared.