If you fly the Corsair right, you should have no problems with either the LA7 or the KI84.
When flying any plane especially the late war American birds in Il2-1946, etc.. get altitude before entering the fight, the Ki84 is not good up high, unfortunately I think the LA7 may be a little uber (russian sim afterall!) but thats just my opinion.
I typically kick the supercharger to stage 2 at about 7400 feet and stage 3 at 14000 feet, if you are unsure, when you switch gears you can hear by the sound of the motor if it is still making power. If it bogs down, then you are too low for it do you any good. I would go into the key settings and set a key for manual supercharger operation.
As always altitude is life, don't attempt to get down low with LA7, KI84 or any other plane that is good at turning especially low speeds. Once you get altitude of at 12-15000 feet, level that big bird off and always keep your airspeed, the Corsair will outrun the Ki84, the LA7 I am not sure since most rooms I fly in or coops don't allow this bird. Fortunately the Ki84 carries very little ammo.
I know many online pilots who fly corsairs regularly and are hard to beat in any plane, in a straight up dogfight.
If you have the option fly the C model Corsair as it has cannons, which gunwise is a great equalizer in terms of armament.
I have flown the Corsair many times and think its one of the best flying, fighting planes in the game if you fly it correctly, think boom and zoom, only turn if you know you can extend and escape. Airspeed, Airspeed, Airspeed!
Myself I prefer the early warbirds, I routinely fly the P40E and Ki43, now you wanna see someone get pissed? Shoot those late war birds down with one of those planes! Of course I am not always successful, well mostly not but when we fly at least two of them together, you can make them guys overwork those bigger, more powerful birds.
Good Luck, if you want I am on Hyperlobby usually on Friday, Saturday night.
Also there is a group of guys who have a room called VMF214 up most weekends, they fly mainly Corsairs, and if you just tag along you might learn something.