jp, the way I would approach this would be to suggest that no matter how good that HS hitter is, he has a hole or holes. When he gets to Milb or college, those holes will be found. He will be forced to adjust or fail. In Milb, the holes get found earlier and exploited far more frequently and consistently each step of the way, with probably the biggest adjustments occurring at AA.
While I expect there are variables, it is much easier to project 92-95 out of HS than it is to project the holes in the swing and the ability to adjust at every level through college and Milb.