I was just wondering, if I replace the stock airfitler with k&n what would I gain out of it?

 

Nothing noticable if that's all you do to it (other than the k&n is easier to clean than the foam filter).

If you run it without the airbox lid and rejet the carb you'll gain a couple hp, but that's still probably not enough to feel the difference. What kind of quad do you have?

 

I have a 2001 yamaha warrior

 

If you get a k&n for your warrior definitely get a clamp-on kit like the one pro-design makes. The factory style k&n filter for the warrior is complete trash, it doesn't seal up right, and falls apart