i have a 06 600rr i bought a 15 tooth sprocket and wasnt very impressed anybody feel this way also, the only thing i noticed was my rpms are alot higher and it jump out a bit better of the line

 

can I ask what you were trying to get out of going a tooth down in the front? I think what you experienced is what a lot of people do the mod for. They drop a tooth in the front in order to gain quicker acceleration which results in a little bit higher RPM. Dropping a tooth in the front isn't a drastic change compared to what can be done with gearing.