My experience after useing MOS since it's beginning as an ervery day system (even in my office):

Every system from G4/ 1GHz (+Altivec) up to 2,7GHz G5 is useable for daily work and 95% of all available applications.
None of these systems "feels" slow. There are not many "power-applications", which takes even a Peg2 to its performance limit.

Only Wayfarer needs more (much more) power - but unfortunately no MOS-supported system can deliver that power today. I mean, it works, but some pages are "pure pain" to load and work (here you recognice, what a "powerfull engine" Webkit is)