as far as results in game play. Players play to their ratings eventually. 21 year olds are not overwhelmed by being in your big, fake park in front of thousands of your fake fans. If they are capable of improving, they will improve at the BL level if they play. They will not wilt from the fake pressure. If a player has BL ratings, he is a fake BL player.
That said, bringing players up affects arb/FA.