Never on 18 October 2015
Assuming it didn't have compatible chips it would typically take a computer about ten times the power. Some emulators can be more efficient but the target would have to be reliable emulation rather than speed so even that would be a great achievement.
It's made harder because it wouldn't be able to split the emulation of a core over multiple cores. So each core of the Wii U would have to be emulated on a single cpu core approximately 10 times as powerful. As I understand it, splitting the emulation of a core over multiple cores caurses issues getting things synchronised that effectively slows it down.
