Hi TM,
You can get k&n filters frm any shop that sells k&n, by that i mean most!!!, but cheapest ive seen is cal sport on ebay who do one for £39.50 plus £4.30 p&p. Regarding yosh pipes you carnt get them anymore in the uk so second hand only and they are very loud!! I know Banditmania do a full race system £199, also ebay, put gsf 400 in the search, but if you want something very special mercuryraceexhausts.co.uk make one offs for the 400supersport racers, these are fully tapered and are the DOGS!!! but costa lota, i read that Jay was going to build some Ti pipes a while back (did you get any done Jay?)maybe he can help, the £-$ is pritty good right now, or if you know someone who can weld stainless JP exhausts at Macclesfield can supply all the separate parts to build your own,ive got an old Beetracing pipe and am going to get JP to bend new pipes when i got some spare wonga, but summers coming so throttle action is top of my list, keep it wide open!!!!