i changed to a 15 tooth front sprocket....didn't like it. i suppose if u ride nothing but street, and don't have many steep grades to climb...the 15 would be ok.
i went back to stock gearing, i take the tw off road a good bit and the 15 just killed it on single track.
i suppose if u could throw a big bore kit in the tw, then it would take more gearing, the way it is.....the lil tw won't take it. unless u live in il. (no hill in sight), and ride street only.