Bear in mind that an engine is a big pump that moves an air/fuel mixture through the cylinder. Replacing the air filter with a high flow filter is a step in the right direction, but it is only part of the solution. In the old days, you replaced the air filter and rejetted the carbs. Today, you have a computerized fuel injection system that requires a program to tell it how to mix that air and fuel. When you make changes to the flow of air, it needs to be told how to react or it won't run right. If you buy a Victory stage 1 exhaust, it includes a K&N high flow filter and a reprogram for the EFI (electronic fuel injection). There are a variety of other manufacturers of exhausts and air filters out there. The key to making it work for you is getting the EFI to run in harmony. The easiest fix is to get all the components from the same manufacturer on the theory that their program will work right. The other is to get the best onboard engine control fuel system you can and play with the settings.