We set up a test Ackermann steering robot, and used a pot on one spindle, and a tie rod connecting both sides together. It uses a window motor and simple linkage to turn the wheels. It was connected to a vex controller, and programming it to run with a joystick for steering seemed to go pretty quickly (CD user TheOtherGuy did the programming), using a PID loop.
I dont know how the mechanical speed of this one compares to your setup....maybe that makes a difference in how sensitive the loop needs to be?