PearPC's long term speed problems will probably lie with the PPC MMU and the prophensity for PPC programs to use Big Endien Data.
The MMU problem wil be solved in time, but learning how PPC software tens to use the MMU, the Byte ordering problem won't be solved until the x86 grows a big endien mode/instructions.
All in all I expect this Emulator to get quite a bit faster, I see no reason to think that 70% of native speed is an unobtainable figure :-D
