But first is true for both cases. If you want to buy something like a Porsche 959, there are only ~200 of them in the world. To get them in the US you have to go through quite the process to have the emissions stuff modified and updated. Even if every one was in the US and constantly driving, I doubt the emissions impact would be worse than a few semi's from the 1970s (and I bet there are plenty of those still on the roads).