Only some of the early -Ns were "faster" -- they sorely missed the extra guns, ammo, and gas so they put them back in place as well as underwing shackles and extra goodies for ground pounding. The majority of the -Ns had this gear and were in fact SLOWER than P-40Es by some 10mph.
I'll quote myself because I already dug up the values and laid them out to show how little difference there was in P-40 performance:
P-40B top speed was 352mph (with 1040hp Allison)
P-40E top speed was 362mph (with 1150hp Allison)
P-40F top speed was 364mph (with 1300hp Merlin 28)
P-40K top speed was 362mph (with 1325hp Allison)
P-40L top speed was 368mph (with 1300hp Merlin 28) *
P-40M was a P-40K but went back to Allison engines (Merlins scarce)
P-40N-1 top speed was 378mph (with 1200hp Allison) **
P-40N-5 top speed was 350mph (with 1200hp Allison) ***
P-40N-15 top speed was 343mph (with 1200hp Allison)
Note the Merlins FTH alt was 19k or so, and the Allison alt was 16k or so. The curves wouldn't be too different, just shifted up. Going from 1100 to 1300hp seems to have almost no effect on this airframe. It was draggy IMO, and couldn't get much faster no matter what engine you put into it.
* = The L was a stripped down version. They removed 250lbs of fuel, ammo, and guns, but all this only netted "a mere 4 mph faster" than the previous version. Other wise identical to P-40F-5 Merlin model.
** = The P-40N-1 had a lightened structure, 31 gallons less fuel, only 4 guns, and only 200 rounds per gun. 400 were built like this. It was only about 10mph faster, yet was the fastest production model P-40.
*** = The P-40N-5 put the guns and ammo back, as pilots complained it couldn't get the job done. It added bomb racks and could carry underwing bombs as well as drop tanks. The extra weight not only dropped the speed back down, but it actually was slower than previous models! This model was exported heavily (1000 to the VVS, and a number to RAAF/RNZAF/etc units). It was used for ground attack and bomber escort missions, but in US service it was only used as a trainer according to a couple of things I've read.
P.S. referring to "most produced" -- they were almost all used for state-side training by the US. One small group in Burma apparently used them, but my hunch is that this was a shipment bound for foreign users and was simply intercepted in a moment of desperation, as the pilots were trained for and told they'd be flying more advanced planes and were surprised to be dumped in P-40s when they got there. While it had many numbers produced, its users were almost entirely foreign countries.
