are you saying he has lost the ability to bowl fast? thats a stretch?When was the last time you watched Steyn? Because anytime I was watching him against India he was firmly in the 83-85mph zone.
if he can consciously change his pace to suit the situation - which is what you are saying - then you are saying that he has flexibility and skill.
and since anderson cant even touch steyns top pace then steyn has more variety, more ability and therefore a more useful bowler.