Would people notice the difference between this and SMP? Probably not, especially on an OS as efficient as AmigaOS.
This matters a great deal.
Many apps these days originated on Linux or other OSs. If these apps can make use of multiple processors they can run faster. These applications are designed for an SMP system, running them on a non SMP-system will get you no speedup because they won't be able to use the other processors. You will have to rewrite any such applications for that to work.
Furthermore any existing Amiga apps will not be able to take advantage of the other processors either. You will have to rewrite them to use the other processors as well. This is no trivial rewrite, it will involve splitting the application into parts and somehow getting them parts to communicate and share data. This is notoriously complicated.
...and I haven't even mentioned Amdahl's law.
The answer to this is very simple. The MorphOS guys had it figured out years ago - you need design and build a new OS. Unfortunately, it's a massive amount of work and will take many years to complete.
Even if applications aren't recoded to actually be multi-threaded in an effective way (but merely ported/recompiled for the "new OS"), having different apps seamlessly spread out running on different CPU's (as well as all the various running OS components themselves) would still mean a general speedup. This is what most people generally mean when talking about multi-processing,
a general speedup by having applications (or threads) distributed to run on all available CPU resources (multi-threading of course improves things even more, by increasing the "resolution" of the distribution). But then we are again talking about SMP, and SMP is incompatible with Amiga!
And the point is - If the cord to the legacy is to be cut, if we are to start with a clean slate (which would also enable the possibilities of 64-bit, true memory protection, etc), then the question arise:
Why continue with PPC at all? You might as well migrate to x86 then, and gain access to real 2013 level desktop and laptop computers at affordable prices! So if they had been going SMP, there would no longer have been any need for $3.000 systems with incomparably poor performance, you could just use real 2013 level HW! This would also be the only way of expanding the size of a shrinking/dying community of users, maybe even to such a degree that it would be relevant to start thinking of the users as a real *market* again, which would in turn also be the only way of attracting new, fresh blood developers, which would in turn generate more users, which would in turn..., ..., ...