hi frode,
i don't know probably on low end G4 will be pretty slow but if the Qemu core will work in another core and it is a qemu-user core i think will be pretty acceptable.
On my Quad G5 for example i have on qemu-system-i386 really good performance with Aros i can play fast sdl games and run there the psx emulation too . Plus i can boot pretty good the X 10.2 jaguar on ppc emulation side.
Im pretty sure on dual core PowerMacs G4-G5 the Fs uae emulation ppc will be really usable and enjoyable .
On my quad for example i have better performances compared with i5 2011 Mac mini ... but we know if not fsuae ppc all can be only suppositions
