If your motor makes more power when you are leaner,that's where it wants to be. In first gear there isn't as much of a load on the motor. Because of this we can run leaner and/or advance the spark timing and/or run more boost. The other thing that allows us to be more aggressive in first gear is the fact that we don't spend much time there.I could pull fuel out of the chip and the car would get stronger and stronger.