Yeager, some info from America's Hundred Thousand:
P-38L engines were V-1710-75/77 and V-1710-111/113. Combat Power of 1,600 hp at 60" Hg was available all the way from sea level up to 28,700 feet. Normal Power of 1,100 hp at 44" Hg was available up to 33,800 feet.
Rate of climb was about 3,400 fpm at 15,000 feet falling off to 2,000 fpm at 30,000 feet.
Speed at 15,000 feet was about 395 mph, increasing to 420 mph at 25,000 feet then falling off to about 410 mph at 30,000 feet.
These are all excellent figures, particularly the climb rate. Compared to other planes, the decline in performance at 30,000 feet was not large at all.