F4UDOA,
The 392mph number is, I believe, the speed the Japanese tests produced using their fuel, 88 octane. Our N1K2 is modeled using inferior fuel, I see no reason to think that the Ki-84 will be any different.
Keep in mind that below 10,000ft the Japanese fuel isn't too much of a disadvantage, it just prevents them from getting good performance at high altitude. Aslo keep in mind that the Ki-84, like the N1K2, will accelerate in a dive very well, which means that a slight altitude advantage for the Ki-84 can effectively mean that it is just as fast as an F4U-1D for the purposes of the fight.