I think it's a lot of money (and the HW is hot and it's noisy), especially since MorphOS/OS4 can only use one single core in a meaningful way. Maybe that single core performs faster than a 1.58GHz Mac Mini, but I actually kind of doubt it. All in all, I seriously doubt it would be worth the effort.

BTW, here are some comments about it, in the context of "new PPC HW":
http://www.powerdeveloper.org/forums/viewtopic.php?t=1739&postdays=0&postorder=asc&start=15