Not even the best P-40 approached P-51 speeds.
XP-40Q was a stripped down testbed prototype... might as well consider it in the same class as the prototypes of the P-39 for all the difference between prototypes and production war-time models.
Three were built, and the design eventually evolved into the final configuration. One was built from a P-40N, and the other two were converted from P-40Ks. They were not stripped down, but were fitted with all Government provided equipment. Aero wise, drag was reduced and the 1,425 (takeoff) hp Allison worked very well. A very similar version would later power most P-82 Twin Mustangs. It could manage 422 mph, not as fast as the P-51, but vastly quicker than the typical P-40.