The P-61 out accelerated the P-47 in a specific circumstance, and out accelerating a P-47N isn't exactly staggering. On WEP and at about 100ft it takes the P-47N 27 seconds to go from 150 to 250mph. It takes the Mosquito Mk VI 25 seconds to do the same.
Also, the P-61A and P-61B, the two that fought in WWII, had a top speed of 366mph.