When you posted "Generally 2 cards in SLI are 1.8 ~ 1.9 times better then a single card of the same sort" - it can only be assumed you meant an sli setup is 1.8/1.9x faster not the theoretical max.

And you're wrong about sli being faster dollar for dollar as well. A faster single card should be cheaper and faster than 2 slower cards in sli.