Performance per watt isn't purely a matter of the process node. You've got architectural improvements to consider (GCN 1.2 being a bit more efficient than PS4's GCN 1.1), potential large power savings by using different memory (GDDR5 doesn't exactly sip power), you could switch to a newer 28nm process (say TSMC's 28HPM or 28HPC+, rather than the 28HP used for PS4/XBO), and finally you could simply use more CUs at a lower clock speed (which will in general use less power than fewer CUs at higher clocks).
Have a look at the R9 Fury X. It achieves substantially higher performance than the R9 290X at pretty much the same power consumption, due to improvements like those listed above. That's not to say that I think NX will be more powerful than PS4, or that Nintendo will necessarily place a large emphasis on power consumption, but if they were determined to, they could probably release a late 2016 console that's noticeably more powerful than PS4 at a similar or lower power draw, while sticking on 28nm.
