Well, I heard on VGC that you'd need a NASA supercomputer to be able to run a Wii U emulator decently within 6 years, so maybe about 20 petaflops or so.
Just kidding. If they go for an x86 architecture, then it would probably need to be a bit ahead of the PS4 in terms of power (using the 360 emulation on the XBO as a frame of reference).