I fitted a K&N filter to my 1000ss last year and it definitely made a difference although I havent put it on a dyno or anything. It pulls harder for longer for sure than it did with the standard filter although Ive also got gorgeous race cans fitted from Two Brothers Racing which Im sure help too!
The noise is incredible if nothing else absolutely fantastic booming sound especially on the over run...
Havent fitted a PC and funnily enough have been thinking that maybe I should as I read recently you can damage your engine if you dont?