S!
At least for AMD it would possibly require a profile to get XFire working at best performance in a title thus they release Application Profiles. Now there is the feedback channel to AMD driver team if you use XFire in Cliffs and get low performance. They did sort out IL-2 issues via that
Also considerwith the patches that have been talked about so there might be light at the end of the tunnel
Cliffs is the first title ever I have considered to be worth CrossFire, if it works and gives me a gain in performance.