Not sure what speeds you were trying to do it at but there was a small tweak to the 38L's flight performance awhile back. They discovered a bug that resulted in the L not getting full power from the engines and very slow speeds (for get the number but it was like 150-100, something like that I think).
Just a little heads up, you will die numerous times when you are getting the hang of it.
Ehh, I'm not a score potato like most of our squad
Statement to the OP.
I seem to remember someone, a loooong time ago, did a test on the turning ability of the various 38's and there was a slight, but noticeable advantage in the J versus the L and the G was the hands down winnar!
There is no noticeable difference in maneuverability between the P-38J and L, the only difference comes into being at high speeds where the L has a better roll rate due to the boosted ailerons. ack-ack