The Jack had a very large frontal section due to its large 14 cyl. 2 row
Kinsei bomber engine. Even if fitted with an extension shaft and close
cowlings, it remains a fat aircraft.
However, the light J2M2 could reach 371mph and the latest J2M5 382mph, which
is faster than your datas. I admit the J2M3 (most produced variant only
reached 365mph).
The N1K2-J could hit 369mph. But its engine cowlings had not been improved
for economical reasons over the N1K1-J.

-One of the problems at this time was petrol availability and quality and
building quality. It is said that a captured Ki-84 (using the same engine
and claiming 'similar' performance : 392mph) running on US hi-octane fuel
went up to 427mph thus exceeding the P47 and P51 top speed.
I presume slightly similar performance could have been recorded with the J2M
and N1K2-J in the same conditions.
-If you thoroughly look at the japanese aircraft appearance in museums, you
would be surprised by the dreadful quality of the surfaces. Those planes
look awful (Cosford Ki-100/Ki-46, NASM A6M5, Pensacola N1K2-J..). Imagine
what could have happened if they had been made with good quality alloy and
qualified workers ? Remember those planes were built in dispersed factories
under the B29 bombs and with MUCH less engineering.
Finally, this was not an bad performance.

It seems like,with available hp the WWII George and Jack should have been
faster (specs on new a/c). Both are commonly referred to as being in the
360
mph class under optimum conditions. Yet they had high hp engines with
four
bladed props. Bad airfoils? Too much frontal cross section? Prop
inefficiency? Weight? (the George supposedly did not have a weight
problem).