> Considering the machines we are used to using, a Cortex-A9 might be adequate.

I don't think a 1.5 GHz Cortex-A9 would be a match for the 1.5 GHz G4 in my Mac mini. A 1.5 GHz Krait maybe, but not Cortex-A9. Of course, this would also depend on whether MorphOS on such ARM CPU could make use of more than one CPU core. But then, if it could, there're also dual-G4 PowerMacs which are already supported by MorphOS with one core. So I think, for me to consider it as an adequate match (or even improvement), the ARM chip must be faster than my G4 on a per-core basis.