That doesn't really make any sense, for various reasons which have already been mentioned.
Notice that the first GT on any console always sells the best. GT3 actually launched with a pretty small userbase, but I don't think GT5 will sell as well. I think it has a small chance of beating GT4 and getting to 10 million, but unless Sony advertises it WELL (TV, Newspapers, Magazines, Billboards, the whole lot), I can see it stopping at 9 million.
Which is still nothing to scoff at.