Im pretty sure most current 28nm vendors could hit those efficiencies if they wanted to
I'm pretty sure it's not that simple or they would all be advertising/doing it.
This is exactly what bitmain did, and when the need arises, so will Cointerra, HF, KnC, Bitmine, BFL, and all the other 28nm vendors. 0.35W/GH at the chip level is nothing special.
If knc could simply lower their voltage why would they spend 10 million on 20nm nre just to get 0.4w/gh at the chip level?
What makes you think the next gen of asics won't be similarly underclockable?