I mean, I believe Fuji using his real power can improve, but not so much, because that's his power, but the others can improve much more, because they are still growing up.

I think I get what you mean (unless I misunderstood what you said, in which case, sorry.) It's because Fuji's a genius, you mean? So he can be pushed to his limit, to play like the genius everyone said he is, but he can't break through the limit of being a genius, because (duh) that's him.

Was in a book Bujold wrote (I'm a big fan): "Perfection doesn't take risks with itself." (Or something like that.) Which may describe Fuji. But the others are not geniuses (prodigies I'll grant you) so it's possible for them to get better and better.