If it's tagged and insured, it's a street car. The only question then is it on street tires - DOT approved is street legal. It doesn't matter if the car is boosted or not - fast is fast.

As to track vs street:
At events like IS, the track gets prepped, so it gets better traction; however, when you have lots of street radial cars running, they kill the surface making it no different than running on a street surface.
The only real reason not to run on the track is because you plan to jump the start and the track would red light you OR you plan to jump out first then hit the brakes before the end to claim that you won. Either way, it only makes sense to insist on the street when you have a track so readily available if you plan to try to cheat.