The P-51A will have significantly less power down low than the Merlin P-51, especially if the V-1650-7 is on board. 200 HP deficit vs. V-1650-3, 300 HP vs. -7, all for WER on 100/130 grade fuel.
P-39N was capable for some 385 mph on WER, with wing guns. The P-39Q, with WER power setting, was capable for around 400 mph when gun pods were not installed. Engine of the P-39Q have had another 1000 ft greater FTH than the engine on P-39N, due to removal of backfire screens (that was possible once the redesigned cylinder air intake was installed).