It was a good insightful post, but it sort of derailed here:
Stop repeating DigitalFoundry on this. The Zelda demo was a quick demo on faulty hardware. Nobody in their right mind would call a 320SPU part even twice as fast as an Xbox 360 GPU. "DX11 level shaders" are also barely different from what is found in the 360 (there's usually just much more of them). It's pretty well established by several people with access to the devkit that the development hardware in the Wii U was a 640 VLIW5 SPU part instead.
That suggests that the Wii U GPU has a fair chance of being more powerful than the Next Xbox GPU, but considering that that chip is going to be part of a SoC, and shrunken down compared to its original process, I think Microsoft is going to clock it at an insane-looking clock speed. An HD6670 like chip is probably attractive to them because of the small die size.