I think the P-38 checks a lot of boxes: Speed? Very good. Including at high altitude. Climb? Absolutely. At least when some fuel has burned off. Turn rate? Quite good, especially with the Fowler-flaps dropped to manoverability setting. Armament? While not that many on paper, they are concentrated in the nose so pretty much devastating. Range? Very good, especially compared to many single-engined fighters. Dive? Not so much: The Mach tuck problems were never really sorted out and the dive brake solution was more of a crutch if anything. So not a very good feature to have given the German's favourite tactic of the split-S followed by a dive.
But then comes the big Achilles heel: Two heavy engines out in the wings. In addition, main fuel tanks in the inner wings. Added to that fuel tanks far out in the outer wings. Sum all of that together and you have a horrendous moment of inertia to overcome when you want to roll it. Compare that to a typical German single with armament in the nose or even synchronized cannons close to the fuselage centerline. Fuel tank under, or L-shaped by the pilot in the centerline. All of which summed together means low moment of inertia and that changing direction by rolling in one direction and reversing into another is easy peasy in the typical German single.
Even Spitfire pilots complained about the Fw-190's rolling and that "turning does not win battles". Now imagine sitting in a P-38 trying to stay on the tail of a Bf-109 or Fw-190 rolling left and right in front of you? This is when that moment of inertia is going to come back and bite you, boosted ailerons or not.
Francis Dean's excellent book America's 100 thousand has a nice section about the P-38 under the chapter "Maneuvering" on page 160 with some pilot's quotes as well which says pretty much the same thing: The P-38's roll acceleration was abysmal.
So I'm certainly with you on the P-38 being a great interceptor, but on the question if it also was a great fighter able to dogfight a single I have to respectfully disagree.