Using a gang scheduler for SMP would allow Forbid/Permit-style disabling of the multitasking to be implemented. Nasty code that uses Forbid to speed things up will have the dubious distinction of being slower though because it will still run on one core only.
The problem is that there is a large proportion of the Amiga's software catalog falls into the "nasty code" category - mostly for optimization reasons. If you implement SMP, you have to break API compatibility. To attempt to have both would at best leave you with a system with (application and probably system) stability somewhat south of Charles Manson.
At which point you may as well just go over to Haiku or some other OS that has these features built into it from the beginning and run UAE.
--edit--
Tbh I think the Dragonfly's experimental internal clustering model would probably be easier to apply to th