cpxxx is onto something, alright.
You will need a bit of height in order to hang with the late-war rides in the MA, so you'll want to take the Ki-61 up to around 10k or 15k altitude or more before you look to engage any enemy. The Ki-61 likes to run downhill, so only engage a higher enemy if you're packing 300ias or more and can get a shot before your airspeed drops below 200ias.
I use the Ki-61 alot during the peak usage hours, and it can really stuff an enemy's advance, even if you're outnumbered, but you should (a) know how you're going to get out of the fight (i.e. run away!) and (b) stay within range of friendlies. If you're going to run away, then you should stay above 10k so you'll have enough slope to dive away.
Last night my Ki-61 was able to chase down an F4U Corsair, force it to go evasive, then outturn it and knock off its tail. Then I chased a Mustang around in a high-speed pursuit, followed it through its vertical maneuvers, and popped it when it pulled up into a high climbing turn. I have to give the 'stang driver credit - if he had kept his nose down, he could've pulled away from me but he wanted to mix it up a bit and that's always admirable.
Next came a 190D from above me. I put my nose down to get some maneuvering speed, then barrell-rolled and forced the FW to overshoot. I followed the FW up into its turn as it looped around over the top and popped it at the apex of its turn.
I chased an LA-7 around on the deck and got within d500 before he got smart and used a combination of turns and throttle to outdistance me.
Finally I was bounced by a Spitfire Mk V as I returned to base. The Spit V played to its strength - turning - but at the cost of losing its airspeed. I went around with it for a turn in lag pursuit but realized I couldn't get inside of it, so I went nose-down and got some separation, acting as the drag for my countryman who jumped on the Spit's tail. When the Spit went evasive with its turns again, I came back around and peppered it with machine gun rounds (I was down to 58 cannon rounds and didn't want to risk wasting them on a too-close snap-shot), sending it into the sea. I was a bit surprised that I could shoot down a Spit with just two machine guns. Something to keep in mind!