i didn't want to make it a straight strenght boost, because it doesn't effect things like off hand weapon strike or damage, or bullrush attempts. it's written the way it is because it's represents the indreased strenght of the one are, not the entire body. also, the carrying capacity boost only applies if both arms are replaced, not just the one.Shouldn't the arm just give a +4 to STR? I can't thing of anything a straight bonus would do that the listed effect does not.
i always imagined everything as being mechanical/clockwork, powered by magic,with movable joints and what not, but it could be tweaked to have multiple possible materials.Perhaps there could be different types of augmented body parts as well, each giving a different weakness and benefit.
what about the balance? does +1 LA work?