How much improvement would I notice with a higer quality air filter like K&N?
Is it worth it? How much do they cost? And Could I install it my self?

Thanks a ton!!

 

With a closed and restrictive stock air filter assembly, don't see any improvement from various magazine articles. An open intake unit will provide some HP gain. K & N website has a number of online resellers and found one selling at $3x range for GS400. It is a stock replacement so just remove the stock unit and insert the K & N filter.