Normal plastic creates a lot more flex than carbon. So any time you're using carbon you're going to take away flex (and possibley grip) from the car. Carbon parts on the b4 are used as stiffeners. To me it seems carbon helps a lot on high bite to smooth out the handling since more flex seems to make the car feel less consistent throughout the course of a corner. I always run a carbon u brace, top plate, and front arms. I'll go back and fourth between rear arms. There are three different stiffness rear arms (regular b4 plastic, b44 rear arm, carbon rear arm) and they are soft, medium, and hard respectively.

As far as shaving the u brace that has nothing to do with this topic. Shaving the u brace lowers your link and in turn raises your roll center. The reason people shave the u brace is because they want to raise the roll center more normally because they are switching to the c hub tower which is 2mm lower than the "a" hub.