There is some very compelling evidence that supports this hypothesis. But the most glaringly obvious is the single PCI power connector. They're not so stupid to design it to handle 420w. But they were compelled to squeeze as much hash out of each chip that they threw away common sense.
With regards to the single PCIe connector and 425W... Understandably, this is WAY over the 150W limit and they WILL be liable if there will be fires because KnC deliberately made this a single connector. Perhaps, they are scrambling now to get a second one in....hence the hash while you wait.
From a practical point of view (try it on your own risk-this is NOT a rec)-many people had run antminer S1 dual blade on Corsair CX500M. This PSU has a single PCIe cable with a split (each half of the split feeds 180-200W blade). When I used it on one of my overclocked S1 (rated at ~400W), the cables were quite worm, but it still run OK, however the machine would not tolerate overclocking further (to 422-435W), so this was probably the culprit. The conclusion: there might be difficulties in running a single PCIe connector at 425W, but up to 400W worked (at least for a short time-I since switched to a different PSU)
I spoke with one of their engineers on IRC about this.
The information he provided was that the 150w listed in the specs for PCI-E isn't a
limit it's the minimum requirement for it to be PCI-E compatible, and that that good quality PSU's can handle it fine.
I'm going the Dual EVGA 1300W route as I have them handy, early order so will report back how it goes.
the problem is not going to be the PSU but the cable/connector from the PSU to the Unit.