The Japanese Zeroes brutalized the Buffaloes, Airacobras and Hurricanes (and Spitfires) early in the war. Also many of the Zeroes kills were against hopeless Chinese pilots and aircraft. So 60 kills for Sakai is not impossible. German Aces sometimes had 300 or more kills. Neither AirForce rotated their pilots out of combat. They flew til they died. So big scores were likely.
Sakai said in his book that the best pilot he ever faced was a Dutchman in his Buffalo. Sakai finally got him after a long pursuit.
He said if the Dutchman had been flying a better plane that he would have beaten Sakai.