Michael is right! When mufflers are removed, it is frequent to observe that air / fuel mixture become too learn and consequently you can lose some HP (even if you win some decibels!) In the worst case, you can destroy your engine
(pistons can reach the melting point). So, if you modify, remove or replace header part (or air filter), you have to tune the carb accordingly (don't forget to check also the spark plugs) to maintain ideal air/fuel ratio. In this case only you can expect win some additional HP.