I don't think so.
it is impressive that a "demo" sold that much and even more than GT4 Prologue, BUT there is a much wider time span between GT5P and GT5 than it was back then with GT4P and GT4. (there also wasn't a NA release for GT4P)
This time people couldn't wait to get Prologue, because the final game was much further away.
I think 8-9 million will be the maximum for Gran Turismo 5.