De Nada, comrade. While my brain has an interesting touch of dyslexia, apparently my fingers are riddled with the malady.
As I read the reviews in the past, that may be true of general I/O and memory operations, but on die, there's no such speed limitation. I doubt you'd see a quartering of real world performance.
A Q630 that can execute PPC-ONLY Code is still a LOT better than one that can't. Remember, Apple made PPC Upgrades for the, half again as narrow, 32 Bit IIci Cache Slot so that the huge installed base of those old DTP Workhorses could do the same trick . . . even more slowly.
Keep in mind: being a Road Apple doesn't make a Mac useless, it just limits its usefulness as compared to the front line Macs of the same era. They're usually a nice speed or CPU-specific code crunching bump from the previous generation's LowendMacs for reasonable price bumps and ofttimes gullies. The prices of low end and mid-range systems reduced significantly over the years in question, IIRC.