Was differential thrust used in the P-38 to improve turn performance?
According to WW2 pilots, no. Aside from the dangers in adjusting 1600hp on one side of the aircraft on the fly in the midst of a dogfight... With the P38, they didn't need to. The P38 had an inherent advantage over single engine prop fighters of its era. Its engines (and propellers) rotated in opposite directions, canceling out any torque effect. The counter rotating engines and props were considered a big secret on the P38 when it was first designed. (as if anyone couldn't look at the angle of the props on each side and tell that.) 

The pronounced torque of the 1600hp-2000hp single seat fighters played a major role in aircraft handling: both in takeoff and power on maneuvers, something WW2 flight simulators never seem to reproduce. Single engine fighters would roll left far quicker than they could roll right, due to the immense torque of the engine/prop combination, aiding in the left roll but inhibiting a roll to the right. This was most evident at lower altitudes, where the air is thicker and the torque effect more pronounced. 

Not so with the P38 - contra rotating engines and props canceled the torque, and the P38 could roll right far quicker than any single engine fighter. While the P38 did have several issues at high altitude, WW2/Vietnam ace Robin Olds held that nothing could beat a P38... down low. (revisit the Dogfights/Air Ambush episode for that) 

So, no, they didn't use asymmetrical engine power for maneuvering with the P38 - they didn't need to. Just roll right and turn - no single engine fighter of that day could stay with you.