Ki-84 was undoubtedly the best Japanese fighter to see significant service and the only Japanese fighter that could, potentially, meet Allied fighters as an equal.
I would not be surprised if the Ha-45-21 powered Ki-84s performed better, in some cases, than the Ki-84 in AH. However, I would be very surprised if any Ki-84 exceeded 410mph in level flight in Japanese service.
If the Japanese industry had not been crippled by the US armed forces and the stupidity of the Japanese government the Ki-84 would have been relatively competitive with most US fighters being employed in the final two years of WWII, if notably less durable to wear and tear. As it was, the Ki-84's potential was crippled by the lack of good fuel, quality control, poor maintainance and lack of well trained pilots.
The Ki-84 was a potentially great fighter, but to say it was decively superior to Allied types is pure hyperbole.