In this case, in all honesty, they actually do mean something. The clockspeed difference is something like 250%. The Espresso would have to literally be 250% more efficient at everything else to be equivalent to the Xenon. It isn't, but it is equivalent enough where it matters.
Spot on.
The Wii U's CPU doesn't need to be as powerful or diverse in feature set as the Xbox 360's and PS3's.
SIMD processing will be handled primarily by the GPU
Audio processing handled by the DSP
I/O processing handled by the I/O controller
I believe it also has a dedicated memory controller(not sure if true?)
Wifi, bluetooth, and data streaming also have their own dedicated processors
The opperating system apparently runs of its own dedicated ARM processor (not sure if true?)
Then there's probably some more i've missed
So compared to the Xenon and Cell processor, the Wii U's CPU wont be tasked with processing audio, I/O, running the background OS, SIMD, etc. So the Wii U's cpu should have a reduced work load vs Xenon and Cell.
Then there's the advtanges of the IBM 750 architecture the CPU is based on. Vs Xenon and Cell the 750 architecture provides better performance per clock, is out of order, has significantly more cache, and with the MCM manufacturing process it should be able to communicate at low latencies with the GPU. There may and likely is other performance advantages as the CPU would no doubt be customized and tweaked with specific instruction sets and features optimised for 3D games.
So all in all, the Wii U CPU doesn't need to be that good. From memory the Cell processor reserves and entire core for running the PS3's opperating system. And the Xbox 360 has known to have an entire thread maxed out with audio processing, and certainly I/O + audio can easily take up and entire thread.