I'm not entirely sure of exactly what happens with attributes, but giving a player 200PA does not mean that they will get 20's across everything. they still have their strengths and weaknesses.
However, I believe that if a player has 200PA and you train them in such a way as to improve, for example, their passing, then there is no reason that, eventually, they would not attain a 20 in passing (assuming of course that he reaches his potential). However, if he reaches his potential (i.e. his CA reaches his PA) before he reaches 20 on passing, then that's pretty much all that he can gain. It may still change slightly over time, but only if he loses points from other areas that he is not training on.
I think that the difference between the CA score and the PA score kind of symbolises how much they can learn. Exactly
what they learn is decided by what happens in their future, with the training they receive obviously being the major factor.
I would imagine that in your example of a player with 100CA and 200PA, he could earn an extra 100CA through training and environmental influences, which would equate to an extra 100 points added to his attributes (That's 100 points on the 0-100 scale as it is in the editor, not 100 points as they are displayed within the player's report in-game), so obviously his attributes as they stand within the original database will form a starting point. If you expect him to develop into a super-human, he's need to be 80% there already, allowing him to train himself to perfection.
Someone may prove me to be wrong, but that's pretty much how I always believed it worked. Hope it makes sense...