There is no such "secret core", I think that's a story from a Neogaf insider (he's trusted on that site, mod approved, take it or leave it) that has been twisted. The insider had claimed that with the first dev kits, developers were having an issue where 1 core was not being utilized and they weren't even aware of it. As dev kits matured, the issue was resolved but supposedly some launch games suffered from this. Again, this is all from what I've read in the technical discussion threads at Neogaf. I tend to believe SOME of the members there as that site seems to be extremely tough on rumor mongering and people making claims out of the blue. A mod confirmed this poster's credentials supposedly. Again, take it or leave it.
As for WiiU compared to the other 2, I'll take the general consensus that I've seen from the many frequent tech threads I've lurked on. What I've come to understand as a good comparison is PC gaming now where a game can run on low or med specs (WiiU) and can also run on high specs (PS4/Xbone). PS4/Xbone games can and should be able to run on WiiU as long as some features are lowered, res dropped, and framerates adjusted. The bigger question will always be whether publishers will want to fund the amount of money into actually doing the work. That's a whole other story/thread.