(typing my notes as Im viewing)
Not really a P38 issue, but watching your reaction to the C2, made me think you could work on judging other planes E. In that instance your closure rate with the C2 was rather slow. Your speed was threenintyish, which means the appearent slow closure rate was due to the C2's speed, not yours. C2, tries to reverse sheds even more speed, hence C2 wasnt really something to react to unless he were pulling a nice preturn.
Coulda used flap looping over on the chog. Not a critical thing, but it may have made for a better/longer shot opportunity. I probably would have pressed the chog for at least 1 more pass (if not longer) since the 51 was many seconds away.
For the 2nd time I noticed you closed on a 51 and stayed pretty much in pure pursuit. Most of the time in that sit they will either roll left or right, and start a turn that eventually goes under. Consider diving slightly lower than them (lag pursuit) before you close to gun range. You have both better visibility, and have your nose pointed where they are expected to go.
You get to the multi-bogies, and zoom over them to about 200mph, dive on the la7, start to zoom, decide to extent for a few, then climb for a few seconds. Nothing wrong with that, but here is a tactic to consider. If you zoom to 150mph, flap down, and continue a ROC 3 down to speeds as low as 110mph. You now can look down, and the enemy with the E state nearest yours should stick out like a sore thumb. Now instead of diving through the whole swarm, you just address those with the highest E as they climb above the swarm.
In the engagment with the 51s&C2, after the pass on the 51, I would have continued in the verticle as described above (because the slow C2 is trailing you). Now here this 51 pretty much says "Im going over the top", you move to lag pursuit. In that case I would have went to lead. The object is the same as earlier, even though the pursuit mode is different. Im trying to get my nose to where I expect them to go. Its much easier to wait for them to come to your nose than it is to try and chase them with it.
Bare in mind Im not saying there's anything wrong with your tactics. You stayed out of trouble pretty well. But you asked about using the 38s advantages. You're not really pushing its verticle ablilities. To do that, you gotta be willing to bank more of your speed into alt in some situations.
