Regular 780 Ti's in SLI put up pretty much the same numbers as a 295x2, so I imagine that watercooled, overclocked Kingpins, would stomp on the 295x2. In addition to that, SLI performs better than Crossfire and will have less or no stuttering issues, whereas the 295x2 will probably have stuttering issues in some games.
The 295x2 would be better for 4K gaming though, but even still you need crossfired 295x2's to be able to play 60+fps at 4K Ultra.
What are you doing with this build? I assume you aren't playing on a single 1080p or 1440p display.