well if potential of a player is SUPERSTAR, then, by buying him we make specific longterm plans on training and a on a way how to manage our teams. If superstar player can be capped quite early, as you say, than there is no need for potential values at all..
Still I cant belive superstar player can slowdown at age of 23 by 200% in training, so I belive this is a bug...
Is there any procedure, how to look in such thing?