I believe highhorse has the cookie! An aileron is essentially a plain flap that can go in either direction. The A-10 with aileron brakes open is essentially a split flap that can go in either direction. Since for any change in angle a split flap creates a greater change in Cl than a plain flap, the split flap/open ailerons will create a greater roll rate than the plain flap/closed ailerons.
H