Yeah but is the performance per watt ratio good?
800 SP. should get about 180 Mh/s or so - for 50 W.
the closest hash rate you can get to that in a desktop PCIe slot is a 5770, for 120 W
the closest wattage in a PCIe slot is a 5670, for 75 Mh/s or so.
theoretically, it looks pretty efficient to me...