A k&n is worth the money and it WONT huet the engine or viod any warrenty. They flow a lot more than stock, you may not notice a big diffence in power but it will help smooth out the power delivery. By adding a jet kit with a slipon it allows you to fine tune your bike. You can gain more topend or more lowend depending how you install it, but a jet kit isnt required. Hope this helps.
2001 ZX-6R
2001 ZX-6R