Actually, the total gain is often greater than the individual numbers combined (assuming you are using HONEST numbers). For instance: If you add a high flow exhasut to a car that is otherwise stock, you will not pick up as much power from the addition of the exhaust system as you would installing it on a car that already had a high flow intake and pullies to induct more air. The more air you're taking in, the more important it is to get exhaust out. This is why it is important to know if an aftermarket part was tested on a stock vehicle or an already modified vehicle, as performance parts will many times yield higher gains on a car that has complemetary mods.