Chennault actually sent a warning of the new Zeros to Washington in 1940 when they appeared in China warning of just how fast and maneuverable they were. No one seemed to pay much attention, as Chennault was something of an outcast. Most of the flying tiger's glory days were fighting small fixed gear and biplane fighters like the Nakajima 96 (Claude?), and most of the Tigers' best pilots quit when they were integrated into the army.
Japanese planes followed a different design philosophy which put more emphasis on speed and maneuverability than on damage absorbtion. As long as they had highly trained pilots, the Japanese could compete. As they lost those pilots, they began losing battles, and as they lost battles they lost the ability to build newer planes and train new pilots. By the end of the war American planes were faster, more maneuverable, ahd more armor, and better pilots, not to mention they had more of them.