Hey cmdrdredd,
How do your 670FTW cards in SLI work on your Maximus V Gene?
The reason I ask is that you mentioned using the 290x DC2 in CFX on a small motherboard. Based on my experience with two open air 7870s in CFX on a Maximus V Gene, I can almost guarantee you it won't work. Even the 7870s had trouble staying cool in a back-to-back arrangement, so dual 290x cards would probably be the kiss of death set up like that.
I assume that externally-exhausting cards like the 670FTW work better in SLI back-to-back, although I haven't tried it.