No chance in Satan's hell it sells the most, my guess is it'll have impressive legs from this point on and will be the second best selling GT game in the series.
My reasons: The PS3's install base is NOWHERE near the PS2's. To sell up to 2nd best in the series would be a feat. Those who think it'll be top - the game market is far more clogged with quality games, particularly for the PS3's case - so I think the anticipation that it'll have monumental legs to get 15mil, is a stretch.