I stop you right there. Bitfury has a chip consuming 0.8 W/Gh/s and it is 55nm. So it is beyond any doubt that a 28nm chip (theoretically 4x more efficient) can increase efficiency to at least 0.77 W/Gh/s (which necessitates only a 1.04x improvement).
I'm not doubting 350W for 600Gh. But, there is a huge difference between a new, efficient chip and simply doing a die shrink on BFL's horribly designed power-hogs.
BFL's are Jalapenos 6W/h Avalon's at 110m are about 9W when overclocked. So clearly the chip design plays a huge role in power use. It's not directly related to feature size like that.