So i ask the question - does it make Kelly a better crosser when Carroll is up there?
Yes, actually. Football is a team sport after all.

Certainly Kelly could improve his awareness of where both the first defender and his target for the cross are, but he'll learn. But one must not forget that a player like Carroll has a better understanding of where he needs to be to pick up a cross for a header, unlike Torres who was always looking to be in a position to get the ball at his feet (a higher risk cross because defenders have more limbs to block a ball at waist height). Kelly's crossing was mainly of the "get it somewhere in the box" type, and that will suit Carroll's elevation more.
So, while your initial statement to the effect that Kelly is poor at crossing the ball is not strictly false, I believe once he begins to concentrate on improving in this area *specifically* because we have Carroll up front (remember that he is a center back brought into the side to give us width for Torres and because we are dire on the left, so crossing for a headed ball would not have been high on his list of priorities). I think he will improve much faster as a result of our change in front man, and therefore "Yes, he is better with Carroll in the side".