At typical MA altitudes performance is roughly the same as a P-51D...for five minutes, at which point it becomes roughly the same as well...a Jug.
M performance is not too far ahead of what P-47Ds were already doing with 150 octane when the M arrived however.
July 24, 1944, 70" MAP was cleared for use by Jugs with the B series R-2800. When tested at 70", it translated into 444 mph @ 23,200 feet. The test aircraft was a P-47D-22-RE Razorback. More powerful B series engines in later D models may have resulted in a bit more speed.

Courtesy of Mike Williams and Neil Sterling
My regards,
Widewing