As for high kill ratio of Mustangs:
That doesn't necessarily establish that Mustangs were better than the Luftwaffe of Japanese offerings. Two things:
1) Luftwaffe: Germany dragged its foot in getting its better fighters ramped up for production. The FW-190A could've been put to production earlier, but they kept trying to make it a bomber. Same goes with the Me-262. Instead of the FW190 Dora, they insisted on using the Bf-109K, which I always thought to be a mediocre fighter.
2) F6F Hellcat: By the end of the war it wasn't the best fighter by any means, but then again it didn't need to be. Japan was still producing the A6M5 en masse, a fighter woefully outmatched by the F6F. In addition, F6F went against Kamikaze planes. Japan in its desperation used slow trainer planes, outdated planes, and even wooden planes (Tsurugi) to plow into US ships. They didn't call the battle at the Marianas islands a "Turkey shoot" for nothing.
Japanese fighters that emerged at the end of the war didn't appear in large enough numbers. And most of the veteran pilots by then were killed--US had a larger pool of pilots so they could rotate them out, whereas Japanese pilots pretty much had to slog through sortie after sortie. If you compare aces, US aces scored the least kills.
**** Bong, the highest US ace, scored 40 kills. By comparison, Iwamoto Tetsuo scored 100 kills. To top it off, Iwamoto did it in the Zero, which by the end of the war was getting badly outclassesed by the F6F and F4U. Most of the kills in a Zero are done by their piddly 7.92mm machine gun.
There were a lot of great Japanese fighters, that had they been produced in larger numbers would've given US pilots series trouble. The Ki-100 Tony (a Ki-61 swapped with a 2000hp Kinsei engine), the Ki-84 Hayate and the N1K1 Kawanishi Shidenkai would give US pilots a heck of a run for their money. Like I mentioned in the other thread, there were a lot of production troubles and part shortages, and the fuel used was of horrible quality, so the planes themselves never saw their true potential.
If Ki-84 fought on equal footing with the P51, I think it'd be evenly matched. Ki-84 had better maneuverability, but could also zoom & boom with its good armament (2x 12.7mm guns and 2x 20mm cannons), making it more verstatile in dogfights.
N1K1 Shiden (you can tell it's not the Shidenkai by its wing structure)
Ki-84