Meh to be honest, those that get a 1k overclock are lucky OR are using LN2.
I get 971 Core on my cheapo PNY (reference GTX 580s), I don't think the VRM itself holds these reference cards back, its just the GPU limit is around there.
But still 24% increase is still good though now matter how you look at it. Odds are though, you can hit this on the reference GTX 580s. Despite the upgraded power circuitry or not, so consider that before shelling out an extra 30~70$. Unless you want the cooler, in which case, it works really well.