Here's a website I found with more info on varients:
http://p-38online.comP38G: new Allison V-1710-F-10 engines; At 24,000 ft. it produced 1,150 hp, and at 27,000 ft., it produced 1,100 hp; It also incorporated the newly designed B-13 superchargers; Late 'G' production models were capable of carrying two 2,000 lb. bombs.
P-38J: V-1710-F-15 engine could produce 1,600 hp. The top speed was 425-430 mph at 30,000 ft., and 406 mph at 20,000 ft.; The leading edge intercoolers were removed, and extra fuel tanks (110 gallons) were installed.; It also had electrical operated dive brakes, and an aileron boost system. This boost system enabled the pilot to only exert 17% of the previous force to control the yoke.
P-38L: V-1710-F30 engine produced 1,600 hp at maximum power, and produced a normal rating of 1,100 at 30,000 ft.; External fuel capacity was increased to allow two 300-gallon tanks to be used; One interesting improvement was the installation of a new tail-warning radar system. It would signal the pilot through flashing lights and bell sounds when an aircraft was in close proximity behind the aircraft. ; Maximum speed at 25,000 ft. was 414 mph,
and could reach 20,000 ft. with a takeoff weight of 17,500 in seven minutes. The P-38 L was the final air superiority/strike variant of the P-38 produced.