As you can see the Major and top AA teams average opponent was almost a AA team while A and B conference teams strength of schedule was much lower.
Now Tim will come on and tell you that the Major players are averaging an opponent lower than Major while an 'A' player is averaging an opponent at his level or higher.
But that isn't the question of course.
The question is if John Smith plays 'A' and then gets picked up by a major team should his stats get better or worse? They would get worse when he joins the major team because the average level of the defense and pitching that he is facing is greater (not to mention the pressure). There are of course exceptions where players being picked up rise to the occasion. There are also players that get picked up and can't get a hit. But for the most part a players stats would be better on a lower level team than that same player playing on a major team.