The P38 was a good aircraft during the early stages of the daylight bombing offensive when there were no other planes to escort the bombers to Germany, however, once the P51 arrived along with more drop tank capability for the P47, the P38s engine freezing at high altitudes and some other flaws caused it to be outclassed by the other aircraft the US could operate. The P38 was not suited for ETO air to air combat, except of course, air to ground work.
Last I checked the 109 compressed at around the same speed that the P38 does, and the 190 is a great diver and has excellent controlability at high kinetic. You must keep in mind that the only P38s the LW faced in large numbers was the F, which was hardly a match for the 109G or Antons, not to mention the D9.
Twin engined day fighters were not needed past 1941, the obsolensence of the Me 110 after the BoB were apparent and the obsolensence of the P38 was realized in late 43 when there were other options for the USAAF.
If the P38 was such a great aircraft, as you stated Citabria--then it would have stayed in service, I beleive that the P38 FM is correct. After all, the second highest-scoring P38 ace, Thomas McGuire was killed from spinning his 38. It was never renowned as a great turner.