Filter kit as in "air filter" or as in "caik" ?
Air filter, some have fitted a I believe "sti panel filter" upgrade and have gained some power, similar is a "green cotton" with same-ish gains
Or go full on with the "cold air indution kit" but a remap would be essential
in doing this !
Or just leave well alone, it seems your well happy
with it in standard form
![Very Happy :D](./images/smilies/icon_biggrin.gif)