(P-38 guy here, flew it nearly exclusively in every sim from the beginning of time..!!)
In my experience asymmetrical thrust does not help much in turns. Think about the thrust line of the engines, when you reduce the thrust of one engine, all you're doing is causing the plane to yaw into the weaker side. You could just as easily achieve the same effect by pressing the rudder pedal.
As an unintended negative consequence you'll actually make your energy situation worse because an uncoordinated / slipping turn is draggy, and also because the full thrust of both engines is not being used to propel the aircraft, your turn rate will actually decrease.
If you want fast turns in the P-38 I recommend staying at the best rate of turn speed, with combat flaps as necessary.
I found that the P-38 can often outmaneuver almost every other fighter while you're both still going fast, and especially in the first 1-3 turns, but the moment the speeds drop below 230ish MPH you will be in big trouble when fighting a smaller aircraft.
If you're positive you can kill him in the first few turns, turnfighting is ok (Just point the nose ahead of where he's going to be and fire the 50's and cannon, then lead the shot once he enters the stream of fire.)
But if you know he's a good fighter or your energy state is already poor, you should consider booming and zooming from the beginning.
As the above poster mentioned, if you dip the throttle of one engine lower for a moment, you can get a snappier roll out of the P-38, but I would advise that the speeds where this happen would be in my "low energy" area, I would avoid doing it unless you're already in trouble. I would use combat or takeoff flaps while doing this too. If you're too fast for takeoff flaps, you're too fast for the roll trick to work anyway.