First of all, think that the developement isn't linear nor proportional, since each player has internal parameters, and these are linked to the player assigned role, in reality it is even more simple, so a player works or he doesn't.

It is like, ok you buy x2 STs, once scores like 25 goals/season in a league where the teams are similar or -10% +10% avq, but the second ST is set to be an assistant, so he'll score maybe 7 goals/season but will provide assistances.
You could buy a 3rd ST or a AML/C/R which are the same, and see how despite train him to 9* he only scores 10 goals meanwhile at the same time you keep the 1st ST, who score 25 goals at 6*.
So the player contribution doesn't fit with the visible skills, in reality the visual skills affect so slightly and these basically impact the beatability margin that sets the match parameters, so to your team avq compated to the opponent avq that can be +17% for you, or whatever.
This sets a distance and the "scenario margins".

So no, you can train all passing you want, you can even train it to 340% but if the player has a limited Dominance power (the power that assigns more or less match animations to every player) and low contributive power (so the effectivity % assigned, like, efficency 35% that a good assistant can have or a 7% that your player can have) this sets a cap.

If you do a friendly vs a team 9-10 levels lower, so a 1% team you will equalize all your players in the same star, the 1* you have and the 9* will be at the same star because the simulator only simulats a distance of 180% and you'll see the power distribution of your players, what they can do and the limits.