If a power supply says it can output 70w isn't that what it can output? I always thought the "efficiency" part applies to how much power it requires (input) to output that 70w. In this case, say it draws 100w to output that maximum 70w, making it 70% efficient. The other 30% is lost as heat etc. Afaik, the power adaptor has to display the exact power it can output.
That's how PC PSUs work anyhow. You buy an 80% rated 700w PSU, it outputs 700w. Not 80% of 700w. Probably draws ~900w from the wall.
And even if WiiUs psu is only drawing 70w and outputting something else, it's likely at least 70% of that (~50w). This is 2013....psu's aren't 50% efficient nowadays.
fwiw: Iwata's comments (when detailing WiiUs hardware) were that the
WiiU uses a maximum 70w, but normal use will be ~40w.
Disclaimer: I'm not claiming to know anything about the subject, just rambling. Probably miles off