Unfortunately, the exact number of the boost still eludes us.
We know that CT will give us a boost, yes. But we do not know how big a boost it is. It is rather inaccurate that a CT will make your level 10 Inside Defender play with the ratings of a level 15 Inside Defender. Instead of increasing your players ratings, the CT actually improves your chances during the play. Your chance of obtaining a rebound or your chance at making a shot. This increase translates to more shots/rebounds made which makes your players to appear to be playing at a higher level.
So if your player has a 30% chance of making a shot, CT might boost it up to 40% (I am just speculating at the amount here). His skill level remain the same throughout the game. It's all about giving you better chances.