Earlier in the war we see fighters with camo, like the P-40, P-47C, P-38G, but by late war they're all unpainted - iconic images of the P-51D, P-47D, P-38L are all unpainted silver finish. Isn't this a significant disadvantage?
While it might not seem like much, paint does add weight and drag, costing a little bit of speed.
It's cheaper and faster to leave aircraft unpainted than to paint them, and the American aircraft industry was churning out planes as fast as possible.
It wasn't that much of a disadvantage. Often, the principle reason that combat aircraft are painted with camouflage is to conceal them on the ground, not so much in the air. The American air forces in Europe had complete air superiority when this shift happened, and this was largely unnecessary. In a similar vein, camouflage doesn't really do much to hide massive formations of aircraft at 20,000+ feet making contrails in the sky.