I actually have phobia of SLI config due overheating problems. I would not go for sli without water cooling....
You've picked my curiosity, just for the sake of argument:
You would rather have two heavily underclocked GPUs in one PCB (like 95% of all dual GPU cards, the most recent exception being the 295X), with a lower rated power delivery system in most cases, cooled by one dual slot cooler that pumps heat back into your case and paying a huge premium for said card as opposed to having two fully featured GPUs, that effectively blow all heat out your case, can arguably be OC way beyond what you can achieve with a dual GPU card, and are a cheaper option than any of these premium dual gpu cards?
Your logic doesn't make much sense to me, unless you are running into space constrains by building a micro atx system....
In any case, I don't see a scenario were SLI cards would benefit more from water cooling than a strapping a water block to a dual GPU card...