A couple of questions which will sway my decision. Is OS4 100% backwards compatible? Put simply, if I run a 68k WHDLoad game from OS4 classic, will it work?
It might. You may have to disable Petunia using a blacklist and run it under an interpreted mode or try EUAE to get it to work. If it does work on the first try it should scream!
There are a few gotchas involved in using the Petunia JIT compiler. I don't think it supports vector base relocation like WHDLoad requires.