If you're considering the usage the current AmigaOS, wich is written in 68k code for the 68k processor, then the answer is yes. But the degree to wich speed is afected depends much on the application. If you're runing only Classic applications then yes of course, because they 're using the 68k processor, and you'll put a faster one there (060). When it concerns PPC code (programs) things get more complicated cause the AmigaOS is still written in 68k code (PPC version coming). In that case, when PPC code is used it all dpends on if the relative amount of calls to the OS. With every call to the OS the computer switches from the PPC processor on wich it was executing the PPC code, to the 68k one to execute the OS function. These switches take very long and harm speed very much, but only on programs that make many OS calls.
Of course when AmigaOS is rewritten for PPC, or if you use a replacement like MOS (different PPC OS but that runs Classic (68k) applications), these switch problems won't happen cause everything will run on PPC, wich can emulate a 68k processor faster than the real thing.
