I seriously doubt that a 5850 uses 85 less watts than a 5870.
I know very little about gpu design but it seems very unlikely that ATI would release a dual gpu 5770 for several reasons. The main one being that the money they would make on it would be a lot less than their other cards. It has nothing with it being or not being a good performer. I'm sure ATI could make a card that worked very well.
I was using the Anandtech review for power consumption but their single card result seems a little off compared to others so yes it's less than that.