lem2: if you compare the turn-in while the car is running, most probably it is on the set-up. if you want to find out the steering response of the servo itself, do it while the car is not moving. if it is slow, loosen the purple nut on the right bellcrank and/or replace the bearings in the bellcrank. you'll be able to determine if there is binding in the steering if you remove the servo linkage.
i use a 9550 servo (it is a bit slower and a tad lower on torque) and a 4deg c-hub but my car turns-in very aggressive. if you know jake de dios, ask him how aggressive my car is...he drove it once