I understand your point with SMP on your existing PPC. But going true SMP breaks compatibility, it's as simple as that. Not only binary compatibility, but also source compatibility. AROS had this experiment called "Silly SMP" which aimed at squeeze in SMP in an Amiga environment. A little hacky IIRC, and not perhaps the most optimal and cleanest way to implement fluid/modern SMP, but it was indeed very close to the Amiga Exec in a way (they had focus on that, rather than some clean-slate implementation using the current best science/knowledge). Small changes to sources and recompile became mandatory. Same with true 64-bit IIRC. The apps must be aware of the new boundaries, old apps assumes the traditional Amiga environment.

With compatibility broken it doesn't make a lot of sense IMHO to stick to the PPC instead of migrating to a modern CPU architecture with modern muscles and peripheral specifications. Better set the compiler to output binaries for AMD-64. Having two parallel CPU architectures going at the same time may risk spreading any remaining support from the few third party developers rather thin. Wouldn't it be better to focus all the new efforts onto a single target platform? But that's only my opinions, and again, who knows...?