Ah but there is a key fact their that you are missing. Those game system are much weaker compared to a desktop. The desktop CPU are have so much more power over the game system that inefficiency in the emulations never show up since the emulated game system is able to be at least as fast if not faster than the native game system even though it is taking lets say 10x more processing power to run the same game.
Emulating hardware is HORRIBLE in efficiency.
Wiis, yes, however the 360 uses a Dual-core 3.2GHz PPC chip. I'd say that's pretty powerful. Funny, MS using a PPC chip.