I think based on what I've learned that the Wii U is closer to this:
360/PS3>>>>Wii U>>>PS4>>Xbox 3
Or like this: Wii U = PC Low/Med settings, PS4= PC High settings, Xbox 3 = PC Ultra settings
Example:
The Witcher 2 PC Low Settings:
The Witcher 2 PC Ultra Settings:
So it's no doubt that the Wii U version of down ported games from the Xbox 3/PS4 will look noticeably worse in a side by side comparison but on their own rights they would still look "good enough" to warrant the time it would take to port it down. Also, if a developer actually tries to port a game to the Wii U and use it's strengths (it's custom features) to "mimic" a certain effect that wouldn't be easily translated from another system's custom features, developers should be able to get very close graphically to the other systems using these techniques.
The Wii U is not going to just be "X" less powerful and that's it......there will be ways around certain effects that wouldn't be possible in the same way on the system with customization.