Aftermarket exhausts that allow more airflow improve performance, not hinder it. But if you improve your exhaust rate, then you also need to improve your air and fuel flow. The right way to do it is to get an exhaust, a high flow air filter, and re-jet your carbs, or retune your injection. Then you maximize all of the essentials all at once and you pull more HP out of the same engine at the cost of noise and emissions.