What Foolkiller said.
When The Speed Channel began covering NASCAR, the gagging was loud and long. I think the same thing would happen in the GT fanbase. I doubt that much new blood would be drawn in by the chance to constantly turn left on those ovals. And as said, I'd rather Polyphony work on getting some awesome drool cars like Ferraris, Porsches - real ones, McLarens, Maseratis and Lambourghinis, rather than waste resources on stuff you can have already in an EA game.
Gran Turismo has been about enthusiast driving of sports and touring cars on a challenging road course. Polyphony can do what they want, but if they do this, I hope I can get those silly NASCAR drift-a-thon events out of the way so I can get back to a real driving experience.