I've just fitted a k+n cone filter , and an round alloy enclosure i made today , similar to the hillpower one . I've used both the original air feeds , acoustic valve left in place .It sounds good , nice roar , pulls well , hard to tell if its better than the standard air box/filter but its definitely not any worse . As soon as its finished i'll post some pics . The surface area of the cone filter is slightly larger than the standard filter also , don't know if this will make a difference or not .