@ tommy hf i know the 3.0 has a froged crand and also the rods seem to be better than the 2.5's.
i under stand your problem, you''l should look at the Z4M if you have te money then, ( as i don't).
only real competitor for the Z4 was a S2000, but choose the BMW for it's name, it's good options and the fact is has a 6 cilinder.