Nod.. like everyone else said, they make a big difference. I gained a fair bit on the top end of my 2000 tomos just by adding that - everything else is strictly stock. It also helps acceleration for me - gets me up to speed a bit faster (which was more important to me than the top end, I never use that in inner-city driving... but a little more power certainly helps in taking off from the lights)
It does make it a big more loud - but thats a plus in my case.. I like the sound!
Makes it sounds more "meaner" :)