The one-sided air combats of Zeros vs. Chinese-flown Polikarpov fighters in 1940 and 1941 created the myth of Zero´s unbeatable combat performance among Japanese pilots, not among Western observers. It also made them to convert more easily from the A5M4 (in which they were used) to more modern plane. In mock-fights the older Claude did beat Zero for being more nimble again and again. Zero was manouverable plane, but Claude was that even more. The mock tests therefore did not convince IJN pilots.
Japanese pilots were in fact quite conservative - initially they liked more of Claude with its open cockpit (good visibility) and fixed landing gears (easy landing and take off) and Claude had even better aerobatic qualities! But when Zero demonstrated its superiority over Polikarpov fighters, which had been challenging opponents to Claude, IJN pilots realised its potential and were willing to convert in more modern "unbeatable" plane.
I have not gone through the records of early Pacific war to see how Zero pilots made it initally against P-40E´s flown by Americans or Brewster Buffalos flown by British/Dutch. The common claim is that Zero and its pilots were superior in all aspects, but I´m not sure how true that assesment is. For example the probable kill/loss -ratio of Dutch Buffalo pilots was not that bad (IIRC, nearly 1:1 against Zeros/Oscars). Buffalo was a better plane than its reputation, when it was handled properly (like by Finns). Perhaps the Allied pilots in late 1941/early 1942 earn more respect than is usually given them. More of this theme:
http://www.amazon.com/Bloody-Shamble.../dp/094881750X
And if we go through the kill/loss stats of Wildcat (USNAF/USMC) vs. Zero (IJNAF) stats, they seem to show that it was about 1:1 during the mid and late 1942 air combats. This despite Zero´s better performance in speed, rate of climb and manouverability. The American Navy/Marine pilot program was not so harsh as the IJNAF program, but still it produced decent pilots who with right tactics could hold their own vs. elite pilots of Japanese Navy.