You'd run into the same problems as you do with WarpOS and PowerUp - context switches that zap performance.
Besides, the main issues with OS4 and compatibility have less to do with the processor and more to do with the substantial rewrite of the OS. It's the same kind of problem we experienced going from 1.3 to 2.04. In other words, even if we had a full 68K build of OS4, the stuff that requires UAE would almost certainly still require UAE to work.
