i'm not sure you'd want 500bhp in a mini anyway, a-series or not. how much of that would actualy be usable? and no doubt a 4 wheel drive system would have to be considered to get the power down.
like i said, rice burner engined minis, especially the FWD, is a quality bit of engineering. although i am not the worlds biggest fan of engine transplants, the FWD bike one has got me very interested. not sure how usable they would be as an everyday road car though. time will tell i suppose, the potential is there though.
as for the reliability issue, alot of this stuff is, i suppose what could be called "pioneering" in that these are some of the first ones ever done, so that will be a case of time telling too.