Looking at Prologue's sales, I know GT will be just fine. I'm not concerned about European sales at all. Japan, if it manages to cross 1 million, that's good enough, and like I said before, the addition of the Super GT license CAN help.
But as for America, Sony needs to heavily advertise the NASCAR section of the game. If they manage to get off on a good start with sales in America, word of mouth should be able to do its magic.
Frankly, when the game releases I'm just gonna check its sales out and how many consoles it shifted and avoid the forums, because I'm fairly certain it'll be way too heated.