would it be worth getting a set US header as they don't have the cat, I've been looking around for a set of tuned headers to go with my Yoshi force GP slip on but as yet nothings turned up.
I would say not.. You can remove the cat material from your own headers by removing system and using a screw driver to work it out. I know others who have done that on here
Assuming you're talking about an RR4/5/6/7 then yes, it's quite a common mod... been looking myself and you seem to be looking at £70 once shipping is factored in. However, a lot of modders say its pointless as the cat in the 4/5/6/7 doesnt restrict air flow so removing it gets you no gains. I've done some searching and haven't found anyone who has dyno tested a bike on UK & US headers with all else being equal.
Yeah mine is the 06 and I was thinking about a smoother airflow rather than knocking out the cat, you would think at high revs a smooth pipe would be better.
I did this mod Tezza.. there is a few pics I posted here a while back at this thread! http://1000rr.co.uk/mods-upgrades-accessories-products/16155.htm I'm not using mine now as I have a full Arrow system, but its far easier to just do this yourself than buy a new Header..
I done this on my RR 6 as I couldn't be bothered to smash cat out 164 before and 166 a year later with us headers but smAshing it out just as good I think