I "had" a stock '99. Per Ace's advice, went with the Mikuni 32mm flat slide. At the same time I fitted an old shorty unrestricted exhaust. Too loud so I put one of those bolt-in silencers on. Absolutely perfect now. It's hard to describe the the difference in performance...it's that noticably improved. For the little money invested, you won't be dissapointed. AND I'm not a roadracer, mostly back roads in the country, but do certainly appreciate the increased HP, top end and responsiveness.
Oh, and when you take off that POS stock silencer, you'll see how heavy it is and can only imagine how restrictive it was...it's killing the bike!