First off, let's keep in mind that the Wii U uses no power-saving features at all.
The console consumes as much while running a 3d game as it does in the menu.
What it saves or doesn't while running the menu doesn't matter. It needs 33W for sustained gameplay, period. Lack of good power scaling won't make a difference for any game that needs that power. Saying that this means it could use much less at full tilt has no basis in reality. PCs have a lot more dynamic range, it's not a good comparison. Almost all of Wii U's power consumption is going to be in the GPU which is going to be pushed hard under normal gaming circumstances.
Plus, if Nintendo didn't do it for Wii U who's to say they can get it into a portable?
As for the measurements at the wall, AC-DC rectifier power supplies usually get around 80% efficiency, whereas a modern lithium-polymer battery gets over 99% discharge efficiency.
You're ignoring the power regulators needed to convert the battery voltage to various rails on the board. That's why the loss due to inefficiency is much less than you think. Since Wii U wasn't designed for battery rails in the first place there's no good bet that it'll fit nicely.
2W for the screens, also too unrealistic. The sizes I gave are actually smaller than the 3DS', so you're saying their current screens are consuming at least 2W.
They're not really smaller.. the sizes you gave are 5" and 4". 3DS XL (NOT 3DS like you said, but it's clear that's what you meant) screens are 4.88" and 4.18". The big difference however is that the screens you specified are much higher resolution. This burns a lot more power for LCDs because a higher percentage of the light is blocked, and thus higher power backlighting is needed for the same brightness. For a good comparison look at iPad 2 vs iPad 3; quadrupling the resolution had a very negative effect on power consumption, you could blame it more or less entirely on the screen because this was even the case when the two extra GPU cores weren't being used.
I don't know what the real figure is, 2W may have been too high but I'd guess at least > 1W.
Then there's the 2 full generations of difference between manufacturing processes. Intel practically tripled the performance/watt between their latest mobile 45nm CPUs and their earliest 22nm counterparts.
I don't know where you got that figure, but even if it's true it's not applicable to what would be a straight shrink.
A much better comparison is Moorestown to Medfield where Intel claimed something like a 45% improvement in power consumption of the CPU core while going from 45nm to 32nm. Intel is going to leverage more than average moving to 22nm than TSMC will from 28nm to 20nm because of FinFETs.
And let's be honest, those are tiny CPUs working at 1.24GHz.
You're going to need most of the power budget for the GPU.
Look, you can stretch whatever arguments you want, you can criticize my numbers (although I stand by > 15W 100%, no question), but it's all an academic exercise because it needs to get under
5W to even be plausible. And that's assuming Nintendo packs a battery that's 3 times higher capacity than anything they've put in a handheld thus far.
If this were in a big 10" tablet form factor then maybe (but that would still be seriously pushing it), but for something like a 3DS XL form factor there isn't a chance in the world.
I don't know why you think this works better as a handheld anyway. The media issue is a serious problem, and whatever game base it has at that point isn't going to have been made with handheld gaming spurts in mind. The asymmetric multiplayer aspect, one of the few selling points, is completely lost on a handheld. So is usage of original Wii-style controls. It'd either need a much better screen than the Wii U controller has or it'll be derided more heavily than it is. Nintendo says they're not making money selling these so it'll need to be even more expensive. It might take focus off of a comparison with PS4 and Durango but instead it'll bring it closer to a comparison with phones and tablets, or the failing Vita which isn't moving in a better direction. People will wonder why Nintendo decided to offer two quite different handhelds and no home console.
3DS support would of course drive the price up again substantially higher.