Changing your pipe changes the airflow of your engine. Since carbs deliver fuel based on airflow, you should rejet anytime you change the airflow be it airbox modifications, pipe, cam, whatever. The correct jetting for your setup will help your power all over, but mostly wherever your airflow mod (in this case your pipe) was designed to improve flow.
It also usually improves fuel efficiency, and you want to do it before any hard running since if the factory jetting is leaner than what you need, you could be melting piston tops.
It also usually improves fuel efficiency, and you want to do it before any hard running since if the factory jetting is leaner than what you need, you could be melting piston tops.