Is it possible to turn the 68K emulation off entirely under OS4?
No. You can remove the JIT emulation by removing or commenting out the line "MODULE Kickstart/petunia.library.kmod" in your SYS:Kickstart/Kicklayout, but it's not possible to disable the interpreting m68k emulator.
Mainly OS4 native apps, which is why the question originally popped into my head. I showed off Cyginx (X11) - running Gimp etc, Universe Two demo, OS4 components (PDF, RawDisk etc..), Hively Tracker got a run out , the latest iBrowse (with Flash plugin), Vice, AVD SDK Browser, MPlayer and DVPlayer with overlay, and a quick demo of how easy it is to construct full size skins in TN.
IIRC, I asked if we could expect a speed improvement if we could remove the interpretive 68k emulator and the answer was "no". So the only thing we could gain doing so would be a smaller kernel file.