The great Japanese Aircraft of the Pacific War by the late René J. Francillon provides excellent insight into the history and development of Imperial Japanese combat aircraft. Japan was hampered by its limited industrial base in manufacturing, raw materials, and quality fuels but its designers had made enormous strides in short periods of time. Yes, they purchased and licensed foreign designs and brought in foreign designers like Richard Vogt (later of Blohm & Voss) to learn the latest techniques. They then applied what they learned to their own original designs that turned out to be quite a shock to the Western Allies, whose racism and intelligence failures led them to conclude that the Japanese had copied Western designs.
I'll never understand why the IJN stuck with the Zero for so long when it was clearly outclassed by 1943 or so and Japanese industry had shown it could build much better aircraft. I guess the early victories and dogfighting culture led to over-reliance on a design long past its prime. A Grumman Hellcat had almost twice the horsepower! Compare a Hellcat to an IJA aircraft of the same period, however, like the Nakajima Ki-84 Hayate (Gale) and the Japanese aircraft is absolutely on par with the Western competition. These specs are from Wikipedia, the Ki-84 numbers from U.S. tests of a captured example, the Grumman numbers from Jane's.
Ki-84-Ia
Empty weight: 2,660 kg (5,864 lb)
Gross weight: 3,601.5 kg (7,940 lb)
Max takeoff weight: 4,170 kg (9,193 lb)
Powerplant: 1 × Nakajima Ha-45-23 Homare 1,522 kW (2,041 hp)
Maximum speed: 682 km/h (424 mph, 368 kn) at 7,000 m
Range: 2,168 km (1,347 mi, 1,171 nmi)
Service ceiling: 11,826 m (38,799 ft)
Rate of climb: 21.84 m/s (4,299 ft/min) at sea level 18.29 m/s (3,600 ft/min) at 3,050 m (10,007 ft)
Wing loading: 171.47 kg/m2 (35.12 lb/sq ft)
2× 12.7 mm Ho-103 machine guns in nose, 350 rounds/gun
2× 20 mm Ho-5 cannon in wings, 150 shells/cannon
F6F-5 Hellcat
Gross weight: 12,598 lb (5,714 kg)
Max takeoff weight: 15,415 lb (6,992 kg)
Powerplant: 1 × Pratt & Whitney R-2800-10W Double Wasp 2,200 hp (1,600 kW)
Maximum speed: 391 mph (629 km/h, 340 kn)
Combat range: 945 mi (1,521 km, 821 nmi)
Ferry range: 1,530 mi (2,460 km, 1,330 nmi)
Service ceiling: 37,300 ft (11,400 m)
Rate of climb: 2,600 ft/min (13 m/s)
Time to altitude: 20,000 ft (6,096 m) in 7 minutes 42 seconds
Wing loading: 37.7 lb/sq ft (184 kg/m2)
6 × 0.50 in (12.7 mm) M2 Browning machine guns, with 400 rounds per gun (All F6F-3, and most F6F-5)
