Actually, torque can actually assist a turn.  The rotary engines on Great War kites were designed to cause the airplanes to turn better in one direction.  However, when you throttle back one engine in the P-38, the resulting loss of power cannot make up for the torque gained.  Power helps you turn much more than torque does.
I suppose that, above corner turning speed, throttling back one engine might help a bit.  But once you're below corner, you need as much power as you can get.