CPU grunt is definately handy for emulation, but perhaps ironically I often prefer emulators on amiga os (or a variant). There's just something about emulation that feels more "genuine" running under amiga os in my opinion. I always put it down to the apis being very raw, ergo the feel, etc., isnt filtered out the way it might be using some other apis.
Amithlon+OS3.x is probably my favorite system when it comes to running emulators, followed by AROS, then Windows, although the reverse is true when there's 3d hardware involved.
Having said this though, it still does fit in with your suggestion that cpu speed is important, even if I personally like amiga based systems for emulation (ie. my "amigas" of choice also have some nice cpu grunt).
Incidently Vice is also available for pretty much every amiga based system out there. Runs great on Amithlon/AROS/MOS, but Id hate to try it on a real amiga