I've talked with Bill @ HPGC, there are a couple of different motors that HPG puts out (different voltage and current ratings), and there are 2 Curtis AC controllers, a 350A and a 550A version.
From what Bill said, the AC-12 and AC-15 motors are different motors, same diameter. One is made to go higher voltage (different windings). This will maximize the performance of the motor for that voltage.