What kind of low speed were you talking about? ~100mph, 150mph, 250mph, ect...?
Definitely under 200. Once you're under 150 and approaching 100, the flaps on the Ki, particularly in the vertical, give it a huge advantage. Most of the really good Ki pilots I played against would try to get the fight slow, then they'd start working the fight vertically. The staying fast part is easier said than done, because if you're converting alt to speed, you risk putting the Ki on a perch. To avoid that, I'd usually give them my six, get them to commit to it, then dive for speed and work around them in the 300 to 350 range. If you don't have enough air below you, or if you're on the deck, that strategy is a bust.
I'm not saying if the airplane turns better than it is superior, I'm saying that if the their best performance is at totally different speeds and they would play the dogfight with different maneuvers. You can't say that one is better than the other, it's like comparing the zero and 190D9.
I think it's a stretch to compare the Ki-84 versus Spit XIV to the Zeke vs. 190D9. I understand what you're trying to say, but you're overstating your case. They're a lot closer in performance than they are apart.