I just believe it to be true because its the one rumour that had always stuck. We've been hearing that since the thing leaked. Obviously it'll get better over time for devs compared to now. But the fact that it needs work to get to the level of this gen consoles isn't so hot.
I don't care about rumours [that for all we know, may have all been from the same few sources].
What we have 'seen' is that it can run current gen games like BOps or AC3 [which have graphics better than most other multiplat games] at least on par, if not even better, with current gen console AT LAUNCH.
Unless Suzuki's team are programming geniuses, if they have hit a wall because of Wii U CPU being lower powered in fact, they are faced with a issue that cannot be solved 2 months prior to console launch if their game already requires all of the PS3/360 calculation power, unless the problem is that they don't know how to harness the power, and not in the CPU capability:
"With the Wii U being new hardware, we're still getting used to developing for it, so there are still a lot of things we don't know yet to bring out the most of the processing power. There's a lot that still needs to be explored in that area."
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However, what which can be reasonably inferred, is that CPU 'relative' to GPU and Ram, is not a clear jump from the current gen. But this is one thing, saying it is less powerful than current gen consoles is something else.
I am not sure why it is like this, but 'maybe' they expect some calculations, like the physics, to be done using the GPU, so they thought it would make sense to make the GPU more powerful instead of the CPU. So CPU may not end up as a bottleneck.