I see that we are having some semantic issues. By "has trained" I mean someone who has done it for a while and thus can have a reasonable sample from which conclusions can be drawn.
Imagine you start playing BB and have two exactly equal 18 y.o. players (same skills and height), one with potential A and the other with potential B, with B>A. If you train them for 2 weeks and only the 2nd one pops you may think that higher potential means faster training. But if you keep training for a longer period you will see that this is not the case and that the players improve at the same rate. That's all I'm trying to say, apparently with limited success :P