The reason the P-38 did well in the PTO and not so well in the ETO is the difference in fighter tactics between the two theaters.
In the pacific, the rule was NEVER to dogfight a Zero. They were too maneuverable and nimble and the American would lose every time. The tactic was to use the US plane's superior speed and fire power and zoom straight at the Japanese plane guns blazing, the fly right by them.
Things were different in Europe. German and allied planes were more evenly matched as far as maneuverability. speed and fire power were concerned. That meant that dogfights were common and the preferred method of destroying the enemy.
The P-38 was a fine fighter, but it was no dogfighter and was better suited to the hit and run warfare of the PTO.
Yet the P-38 performed well in North Africa and in Italy. The Italians had great respect for the P-38 and the Luftwaffe pilots in the Med theater were not fond of it either.
No there was something in the ETO that limited the effectiveness of the P-38. I suspect that the British av-gas may have been a large part of the problem in the use of the P-38 in the ETO.
With the addition of the maneuvering flaps the P-38 could almost match most any single engine fighter in single air to air combat.
Not trying to take anything away from the Lightning as it is one of my favorite aircraft.
Tactics is a valid point, it was the use of the high speed dive and zoom climb that enabled the P-40s and F4Fs to outfight the Zeros in the PTO. So why not use the same tactics in the ETO
Regards
alfa6 ;>}