Read this good links, I tried to bring the major differences between J and L together as you asked:
As far as I know the P38J had nearly the same specifications as the L, both came up to the same engine rating and the designed was not changed very much. The one major problem which broke the necks of most P38 pilots until the L versions was the leaking dive abbility. At some point after 400mph (most pilots didnt dare to find out were exactly it was...) The air was so much compressed by the cockpit and the wing roots, that the airflow was directed over and down the elevator, so due to this lack of airflow at the elevator the pilot lost totaly pitch controll at that point. Unless they havent tried to bail out (something that was also known to be very tricky in all 38's) there was nothing to do than throttle down and pray the compression would slow down the bird good enough to get below 400mph and get back the pitch control (most of the times unlikely). Some late versions of the P38J (think it was P38J-25-LO) and all P38L got extra developed dive flaps to bring back the airflow to the elevator to avoid this nasty dive effects. Namely for this development the Luftwaffe dedicated downed shortly after it a transport aircraft on the way to brittain with a full load of this extra dive flaps supposed to upgrade some P38J's squadrons... Another addon of the P38L and later P38J (-25-LO) modells was the hydraudically boosted ailerons which used to push up the roll performance. So if HTC is going to implement the P38J some day I expect it to be nearly as fast, climbing and turning like the L, just with a slower rate of roll and this above 400mph dive problem. But well if they wanna do the P38 fans a favour they could keep the modell of the P38L repaint the skin a little bit and call it a P38J-25-LO.
Anybody feel free to correct me or add other facts...
