Let's see here:
Genesis - 68k has been OC'ed to 2x speed.
GBA - same deal with its ARM7, just hasn't caught on yet...
Xbox - has had 1GHz+ PIII/Celerons modded in....
N64 - I believe the multiplier can be broken wide open for a 33% CPU clock increase...
PSX - both the audio & primary CPU have been overclocked to 2x speed (that's 25MHz and 67MHz, respectively)...cuts down load times and slowdown in games
Of course the GP32 can be easily doubled from its 66MHz base, and some can be raised by a factor of 2.5 times. And it's got the added benefit of having it all done software-side.