A 580 is currently power limited due to spiking above the total available power of 150+75+75=300watt max. In tpu's article
here the 580 power limits itself after spiking up then it settles to a max watts of 200(question why 200 and not something more like 250 or 290??? Probably due to the limits of the electronics used on board...). One question I didn't see answered was did the performance increase proportionately to the increase in power used after the power limit was taken away?
In any case the point is a 580 really needs an 8pin+8pin power to keep itself from being power limited from the cables(let alone the electronics used on the board as well as possible problems with load balancing), so how is a dual gpu going to fare better if a single gpu is already power limited? I guess I will just have to wait till someone who actually knows what they're talking about gives a go at it, or just wait till the card comes out and reviews are posted.
ah! My guess: The gtx580 is so powerful to the point it has to be power limited when used to its fullest, however most applications will be limited by the weakest link in the gpu before using the full power of the gtx580(example: cut the rops in half and suddenly the card is limited by the rop count). The dual gpu card will still need to be power limited, but it won't matter as most normal applications will be limited by the weakest link in the gpu before reaching the power limit.