I don't believe it is accurate to say a high end PC can only barely emulate PS2 games, nor do I think it's accurate to say that we don't know much about how the ps2 works...
The pcsx2 project has had huge progress in the last few years, even laptops can usually play just about any PS2 game at native resolution. If you up the internal render resolution to match native screen resolution, then yeah you'll end up needing a decent GPU for it, but standard PS2 emulation isn't nearly as CPU heavy as it used to be. I've got a couple year old 15" MacBook Pro (core i5) that can handle any game I've tossed at it at native res, and can even handle some of the games at 2x and 3x internal render res with minimal hiccups. I'm not saying this means that it will be easy to emulate on the WiiU, just that the comments so far about the state of emulation for the PS2 didn't seem to be correct vs. what I had experienced.
Admittedly, I did documentation for them, so I may be more familiar with the emulator and how it works and how to set it up than most, but all in all, I don't have too many complaints with its performance.