Actually, I think the improvements to the 68k emulator were incorporated into later releases of the system software/Mac OS, so simply running a newer OS will allow one to enjoy the benefits of a faster 68k-on-PPC experience on any x100 Power Mac (and also reap the rewards of said OS having more PPC native code, speeding things up further).
If one wanted to run the stock OS (System 7.1.2 or 7.5.x) on such, however, SpeedDoubler is probably a good idea.
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