Increase In power due to the positive gas exchange work
In the case of the naturally aspirated engine, the suction pressure is less than the exhaust pressure. So, a negative work is done during the gas exchange process. In the supercharged engine the inlet pressure is higher than the exhaust pressure, therefore, a positive work is done during the gas exchange process. This is because, during the exhaust stroke the piston has to move only against the back pressure in both the cases whether the engine is naturally aspirated or supercharged, but during the suction stroke the pressure is higher in the supercharged engine, hence there is additional energy delivered to the piston and therefore more power is obtained.