Entropy, I would deposit this to chew on: We have already seen multiplatform titles that "Console A" is locked at 30fps while "Console B" is at an average 28fps (drops/tearing) with a reduction in transparency resolution, texture resolution, disabled graphic features, etc and even sometimes reduced render resolution -- and the normal consumer really cannot see the difference. And while I am not a developer and just an longtime PC gamers I would say that in the above scenario "Console A" and "Console B" are more than the ~7% difference in performance for the app--if Console B had all the same settings the framerate would reduce much more and unlocking the 30Hz Console A version would also open up too.
All that to say that if the WiiU is *very* close to the 360 that is not a big deal. And it sounds like the door is open for the final dev kits and the retail versions to be slightly better than the current builds so something that is same-to-2x as fast should not be a huge issue on the graphics side for 360 ports. Even if, big if, the retail version was 95% as fast on the GPU I am sure it would be easy to make that gap dissappear and the features disabled would not be noticed by many. I am clueless, but I would bet the WiiU ends up being slightly faster.
This generation has seen the PS3 and 360 get the same games with the relatively minor differences you point out. But then again they were released with roughly the same transistor and power budgets, and were designed at roughly the same time, even if the PS3 were held up by the blue laser availability/yields/price.
The Wii U, coming to the market 6-7 years later, can avoid those porting issues easily,
if it is a priority at all. Looking at the processing density of AMDs later products, matching the 360 in terms of shading power is in the ballpark of 20mm2, or (ballpark again) $2 added to the bill of materials. Put another way, if you are at 360 level of graphics calculations, you can double it for $2. So unless Nintendo is extremely cautious in terms of cost, I think it can be safely assumed that the GPU will have some headroom vs. the PS360s.
Again, as an exercise, you could turn the argument around, and ask yourself just what it would mean if Nintendo did in fact produce a console that had a weaker GPU than the PS360. It would mean that
a: Nintendo isn't interested in ports from the previous generation, much less future multi platform titles.
This is conceivable - after all, this is exactly what they did with the Wii, and now with the 3DS.
b: Nintendo is only paying lip service to being a HD console.
The PS360 has issues even performing at 720p, a resolution which by now is basically gone from dealer shelves in other consumer products. Targeting anything but 1080 output seems bizarre for a new console, but if it has lower performance than the previous generation, the WiiU will have difficulty doing even the half resolution format. Of course they could simply scale SD output to HD resolution, but the TV sets already do this, so why commission IBM and AMD to design new CPUs and GPUs for the WiiU at all?
Nintendo certainly knows the importance of hitting the right pricing spot, and the 3DS definitely drove the point home. But even so, I can't see them coming in below the previous generation in GPU performance. It just doesn't make sense, cost cutting that deeply simply isn't worthwhile. And what people saw in the "Garden" demo, doesn't really imply sub-par performance either. In the absence of actual specs/target specs being leaked, Occams razor is probably just as useful as any vague opinion offered by insiders. What does "arbitrary number" more powerful than the 360 mean anyway? In terms of what? Number of CPUs? CPU clocks? CPU marketing FLOPS? CPU internal benchmarking? L2 cache size? L2 cache bandwidth? GPU shader power? GPU shader FLOPS? GPU bandwidth? GPU EDRAM size? GPU internal benchmarking performance? Main memory size? People dishing out such an arbitrary number either don't know what they are talking about, or they don't want the recipients to know what they are talking about. It's completely useless.