No one will actually know until KNC does at-the-wall tests on a real unit hashing.
My two bitfury cards pull ~ 46 watts at 66 Gh/s
It wouldn't surprise me if a 55nm full custom design turned out to be more power efficient than a 28nm standard cell design. I would assume that it's possible to under clock the KNCminer chip to get the Watts/GH/s down. Looking at the cooling hardware they have used, you can see the KNCminer chip is extremely overclocked, they are trying to squeeze the most out of it at the expense of efficiency. It would have made more sense to use twice as many chips at half the clock rate to get the power usage down, but that means they could only build half the units per wafer. In the end the machines left mining will be the ones that use the least Watts per GH/s. There is no room in mining for overclocking, else we would all still be using GPUs and ASIC miners wouldn't exist would they. It's all about Watts per GH/s.
