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.
I won't spend more on a bike than I think it's worth, but if it's a good deal, I don't seem to have a problem buying bikes I don't need.
2002 Vulcan 750, 2013 Royal Enfield B5
2001 XT225, 2009 Genuine Stella
1980 Puch moped