Personally I'd go ATI for DX11, the fermi set up is just not that great, way over hyped by nvidia with very little put out.
the 5k series is more than capable, even if ATI's drivers have been and are a touch shaky, then again I've herd very bad things about the 4xx fermi drivers
SLI gets, given 100% = your cards performance, thus 200% would be what you'd expect from two cards as a laymen, about 130% to 150%, closer to 138% in my experience, same for crossfire. (Then again i'm pretty old school, this is possibly even a year out of date info by now)
I'd recommend one GPU over two in any case, and give two GPU's at $200, vs one GPU at $220 (in example) I'd still go for the one over the two, even with that pricing issue.