The more boost the ECU sees the less the timing it will advance. The higher the Charge Air Temp the less timing it will advance. Both work conterproductivly with a larger pully from a factory tune perspective. Yes the car makes more boost and yes it will make more power (good rule of thumb would be 7-12hp per psi boost) but at the same time your going to lose power by running less timing. So in the case of a factory tune I could see where the power lost by the reduced timing is larger then the power gained by the increased manifold pressure.

The trick is to win in both worlds.