Many of the ricers around here have it hooked up on street cars (these are the same guys who blow their mostly stock engines at the track), cops don't know or care, but you can't use it. If a cop so much as hears you squeal the tires leaving a light, you are going to get busted for reckless driving.
In 45 years of riding and working on motorcycles, I have yet to find anything better than stock. When you make engine modifications to a bike, it seems you always lose way more than you gain. I did go up one tooth on the front sprocket on an '07 Ninja 250, that bike was seriously undergeared. Some went way more than that, but I found that the 15T sprocket (instead of a 14T) made it perfect.