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.
Personally, I think the FPGA approach to expanding the Amiga (eg. making slave processors to the master single-threaded CPU) is the best way but SMP is certainly possible.
