The only official statements concerning the power of the system comes from Iwata, according to him the best looking Wii U games shown at E3 only reached about halfway of what the console could have outputted. He has also mentioned that the Wii U is designed to compete with future game systems, not current.
Leaked spec-sheets says about 4x the 360 which is pretty much what we should expect. Remember, 720 is rumored to be about 6x more powerful, this means that the gap between these consoles should be insignificant.
Finally, developers who are actually working with the console say that they're very pleased and that the Wii U is a true next gen system.
In any case it's practically impossible for it to be on par and even less for it to be weaker that current gen.
Wii was a one time gamble, Iwata regrets their decisions and he wants to correct what went wrong with online and hardware, casuals have moved on and thus according to Nintendo won't/can't be their main financial source anymore.
Can't believe what impact the bullshit article about the two anonymous developers had, the rumors even made it to my local newspaper where they stated that the Wii U wouldn't be comparable with the 360 even, wtf?
There's a huge amount of positive and credible sources out there yet people still want to believe and spread this rumor?
Sigh...