Speculation is nice and all, but it doesn't lead anywhere. Even if (old) this rumour was true, the only thing we would know is that the Wii U GPU is RV770 based, which is barely better than nothing. We still don't know the actual specifications (clocks, EDRAM, SPUs, TMUs, ROPs, etc.) so any attempts at determining its performance level now are basically pointless.
Even if we had those details, we still wouldn't know how the multiple screen rendering will affect performance. It's fair to say that those expecting multiple Wii U controller support are going to end up being disappointed, and so will be those who are expecting the hardware in the WiiU to be vastly superior to the other current generation consoles. Given the size of the box, the power envelope used for the various components, and Nintendo's penchant for day-one profitability, they can't aim that high. Those expecting a box with a Power 7 CPU with a top of the line RV770 GPU are clearly in denial.
The sensible thing to expect is a two-year old mid-range GPU and a Power 7 based custom CPU downclocked and with less cores. Nintendo only needs a bit of headroom to achieve performance parity with the other consoles while being able to stream to the controller. After all, it's not like, this far into the generation, third parties are going to suddenly choose the Wii U as their lead platform, which is the one thing that would make the Wii U versions better. When it comes to Nintendo, their games have always been about artstyle more than technical prowess, so they don't need to have state-of-the-art hardware either.