cant comment on the sp as im running pipercross (or was before i binned it lol) but on the firestorm it ran like a bag of fk with the k&n so i swapped it out for a bmc race filter - 1404r iirc
had to go from 180/185 jets up to 205/210s lol.
massive airflow and no flat spots.
good luck finding them though - i got mine from a seller in italy.
if you get stuck finding them post back and i'll go through my paypal account and find their addy for you
was same price as the k&n though - £58 delivered