I don't think you are making a fair comparison. You say it runs cooler compared to a 5970 but the 5970 is just air cooled while you are comparing it to a watercooled GPU. Also SLI only offers a improvement in games that support it and for double the cost you certainly don't get double the performance.
Furthermore, you say it is cheaper but you don't factor the additional cost of all the watercooling equipment you need as well.