Nissan Skyline and Toyota Supra and illegal in some states because of their emission levels, kinda stupid but if they don't make it, then they're illegal. If you're talking about the Nissan GTR than probably not, this car should not be illegal, the cop thing could be true for this car.
Also Supra cannot compete with a Viper, Lambo, Ferrari. The've got 8-10 cylinders and supra is only V6 (twin turbos). Supra is fast only when you're comparing to other tuner cars like Sentra SER Spec V, Skyline, Civic SI, RX-7, RX-8, Evo, Subaru. Supra is a fast car, boring interior but its actually worth to buy, if your into speed and nice cars.
Good Luck!!!!!