I disagree. Once the exhaust is opened up, you'll get even better results with additional mods. Remember, just cause the box says "Bolt on (up to) 20 HP", it usually comes with a disclaimer that actual gains may vary, and are subject to additional modifications. That'* the one thing that'* always bugged me about this hobby. The ads and exaggerated promises of cheap horsepower.
Maybe I am wrong, but I think you need to let the engine breath better before you can start expecting the other things to make real power.