There's not that big of a difference, have you checked out the review? The gtx beats the gts by only a few gts, meaning that its due to only the clock increases (which includes the extra memory bandwidth, due to the smaller memory interface than the old gtx, the extra bandwidth makes a big difference).

But I do think there's a few things nvidia is hiding from us, for instance, the power consumption is something like 40w greater at load than the gts 512 and it needs a second power connector. The small clockspeed increase alone can't add that much extra power consumption, the only possible explanations are either that engineering sample was an early modle and consumed a lot of unnecessary power, the 4 phase power supply needs some more power but allows for further overclocking (and I believe the stock vcore is higher), or the gtx has some feature we don't know about yet that slightly improves performance but adds to power consumption