Mna, the Ki-84 is a great plane, very similar to Spit VIII, though, perhaps, the VIII has an edge on it (Widewing has always stated that, unfortunately we live too far away, we couldn't have a showdown in DA!

Just do a search on the BBS, there were several Ki-84 thread).
It's a bit nastier when it stalls than a Spit, but it's a very good furballer. I never turned off Combat Trim, and I never felt the need to do so. Anyway, many other pilots, like Karnak, prefers to disable it... your choice in the end.
Another thing: it's a Jap plane, that means poor protection, especially for the pilot, it's often the first red entry in the damage list.
It likes to fight nose up, mostly because the flap can be extracted only at very low speed (<170 mph, the second thing I hate about it) and, without them, the plane is hard to control in knife fights.
Karnak, may I ask you what you mean with "durability"? And when you say that one of Spitfire advantages is "firepower"? I'd say better ballistic, on the raw hitting power side I'd say they're equivalent, Ki has a good punch.
EDIT: corrected a typo, thanks for the quote, Lusche, I saw it there! ;D