Finding reliable specs on Japanese planes can be difficult as there is a lot of conflicting info out there, at least in my experience. Check three different sources and you're likely to find three completely different sets of numbers.
One of the most thorough and well documented sources I've found online is the series of articles written by Joe Baugher. From his article on the Ki-84:
-----------------------------------------------------------------------------------
Specification of Nakajima Ki-84-1a:
Engine: One Army Type 4 eighteen-cylinder air-cooled radial (Nakajima Ha-45). The following engine models were used:
[Ha-45]11 rated at 1800 hp for takeoff and 1650 hp at 6560 feet.
[Ha-45]12 rated at 1825 hp for takeoff and 1670 hp at 7875 feet.
[Ha-45]21 rated at 1990 hp for takeoff and 1850 hp at 5740 feet.
[Ha-45]23 rated at 1900 hp for takeoff and 1670 hp at 4725 feet.
Performance (early production):
Maximum speed 392 mph at 20,080 feet, cruising speed 277 mph.
An altitude of 16,405 feet could be reached in 5 minutes 54 seconds.
An altitude of 26,240 feet could be attained in 11 minutes 40 seconds.
Service ceiling 34,450 feet.
Normal range 1053 miles, maximum range 1347 miles.Weights: 5864 pounds empty, 7955 pounds loaded, 8576 pounds maximum.
Dimensions: Wingspan 36 feet 10 7/16 inches, length 32 feet 6 9/16 inches, height 11 feet 1 1/4 inches, wing area 226.04 square feet.
Armament: Two fuselage mounted 12.7-mm Type 1 (Ho-103) machine guns and two wing-mounted 20-mm Ho-5 cannon (Ki-84-Ia).
Two fuselage-mounted 20-mm Ho-5 cannon and two wing-mounted 20-mm Ho-5 cannon (Ki-84-Ib).
Two fuselage-mounted 20-mm Ho-5 cannon and two wing-mounted 30-mm Ho-105 cannon (Ki-84-Ic).
External stores included two 551-pound bombs or two 44-Imp gallon (200 litre) drop tanks.
Sources:
The Nakajima Ki-84, Rene J. Francillon, Aircraft in Profile, 1969.
Famous Fighters of the Second World War, Second Series, William Green, Doubleday, 1967.
Japanese Aircraft of the Pacific War, Rene J. Francillon, Naval Institute Press, 1979.
War Planes of the Second World War, Fighters, William Green, Doubleday, 1964.
-----------------------------------------------------------------------------------
Judging the performance of a plane by its flight model in AW(or any other sim) isn't the best way to get a picture of its real life capabilities. While it's possible that a given FM may be very close to a planes actual performance, it's just as possible that there could be large errors or intentional "game play" tweaks built in to the FM. It just depends on who is doing the modelling and how good their information is.
That being said, we could draw some conclusions on how the Ki-84 might perform in AH by looking at the N1K2-J. From Baughers article on the N1K:
-----------------------------------------------------------------------------------
Specification of the Kawanishi N1K2-J Shiden Kai:
One Nakajima NK9H Homare 21 eighteen-cylinder air-cooled radial rated at 1990 hp for takeoff, 1825 hp at 5740 feet, 1625 hp at 20,015 feet.
Performance: Maximum speed 369 mph at 19,355 feet, 359 mph at 9840 feet.
Cruising speed 230 mph at 9845 feet, service ceiling 35,300 feet cruising speed 230 mph at 6600 feet.
Climb to 19,685 feet in 7 minutes 22 seconds.
Normal range 1066 miles at 219 mph at 9840 feet, maximum range 1488 miles with 88 Imp. gall. drop tank.
Weights: 5858 pounds empty, 8818 pounds loaded, 10,714 pounds maximum loaded.
Dimensions: wingspan 39 feet 4 7/16 inches, length 30 feet 7 29/32 inches, height 12 feet 11 29/32 inches, wing area 252.95 square feet.
Armament: Four 20-mm Type 99 Model 2 cannon in the wings. Two 551-pound bombs or one 88 Imp. gall. drop tank could be carried externally.
Sources:
Famous Fighters of the Second World War, William Green, Doubleday, 1967.
Warplanes of the Second World War, Volume III, William Green, Doubleday, 1964.
Japanese Aircraft of the Pacific War, Rene J. Francillon, Naval Institute Press, 1979.
Japanese Fighter by George! Robert Mikesh, Wings, April 1995.
-----------------------------------------------------------------------------------
If we compare the N1K2-J to the Ha-45 model 21 powered Ki-84, (unless I'm mistaken the Ha-45/21 is basically the army version of the same NK9H Homare 21 in the N1K2, please correct me if I'm wrong. In any case, rated power is nearly identical.) we get these numbers:
Power loading(normal loaded weight at SL):
N1K2: 4.43 lb/hp
Ki-84: 3.98 lb/hp
Wing loading:
N1K2: 34.86 lb/sq.ft
Ki-84: 35.19 lb/sq.ft
This suggests that the Ki-84 would climb and accelerate a little better than the N1K (which is no slouch in those categories) while the N1K would have a slight edge in sustained turning ability.
There have been charts posted here recently that showed a SL speed of about 350mph for the Ki-84. If those are accurate, then it is approximately 20mph faster than the N1K2 on the deck, as well as at 20k. I would guess it holds a similar advantage at most altitudes.
So...a cannon armed plane that is 20mph faster than an N1K, climbs and accels better, and turns nearly as well. Sounds yummy!! unless it's on yer 6
