So how well does 680x0 code run on a 1.25Mhz G4?
Very. MOS uses JIT emulation, executing 680x0 code in a manner similar to UAE. Unlike UAE, however system calls are inevitably routed through native code, which can only help.
I've found that my 800MHz G4 with Petunia (OS4's JIT) is far faster than any of my real 680x0 machines for all my code/apps. From what I gather, trance (MOS's JIT) is even more aggressive.