Theoretically, ShapeShifter should work at full speed using the OS4 68K JIT, since ShapeShifter is more in line with today's virtual machines like VMWare and Parallels than a true emulator, since it uses the host's own CPU as-is and just emulates the additional bits. If it doesn't work, it's probably due to limitations of the OS4 JIT (it doesn't emulate everything). The 68K BasiliskII might be a better test under OS4, since I think it's a little "cleaner" on the hardware-banging side (and also doesn't emulate the CPU).
Point being, there could also theoretically be a version of 68K UAE built the same way - use the native CPU instead of emulating. I'm not sure how big a project that would be. Maybe adapting the in-program JIT for PPC is easier!
