As both a Crossfire and Xfire user as well as someone running hardware monitoring around the clock i can probably clear this up pretty well i hope
Firstly, SH does NOT use Crossfire. My Primary card will run at 100% much of the time, but the secondary will NOT be used ever in SH.
Secondly, Xfire works fine. The only problem is if you bring up a chat window with the mouse unlocked moving the mouse still moves the camera, even with the overlay up. Just gotta make sure you can see the mouse cursor and your golden. Much better than having to Alt tab to talk to someone as SH is notorious for bugging out when the player does that.
As far as your framerate is concerned i can't help you much. I'm running 2x 4870's which are far less powerful than your 5970. Only thing i could recommend is checking your CCC and turning AA down if it's really high. 4x is usually enough. 8x and 16x are practically unnoticeable. The spray effects are quite intensive as well i've found. If your looking at the spray coming off a ships bow from close range you'll suffer a large drop.