A screaming eagle air filter is great but if it's a carburated motor you will need to rejet the carbs or it will run lean causing misfires and burning through the top of the pistons.
The Screamin' Eagle Stage 1 Air Kit isn't about the filter as much as it is about the new backplate that's with it. This increases air flow = more power.
I assume you are talking about the K&N replacement filter available for Harleys. IT is a much better filter than the crapper filter that comes on there bikes. It is reusable and won't void the warranty and it should improve air flow slightly.