P38 could, and can in AH, outturn 109s. What I think you are doing wrong is comparing the J/L to E and F. P38G wouldn't have been fighting the E version often, but the P38G can outturn the 109F. I have no trouble outturning 109G2-109K4 in a J or L. Sometimes a good pilot will give me some trouble but the trusty ole P38 can fly slower than the 109 and once I get behind a 109, if it tries turning will soon realize it's not getting anywhere and will use it's speed to get away. Seems a lot like real life to me...
The P38 doesn't have to have alt to win... but like i posted earlier, a lot of people that think they know the P38 think that is how they have to get kills. Hell if I encounter a TnB plane in the MA and there are no other enemies around, if on the rare occasion I am higher, I burn E until it is co alt and hope they don't run.