One thing is for sure, GT5 will have a very hard time to break the 10 million barrier, and if it will it won't be by much. It's not only that Playstation 3 is smaller than PS1&PS2, it's also the fact that the care game genre isn't as big as before.
The problem is to predict Halo: Reach. Is Halo on a decline? Will Reach be able to outsell Halo 3 or not? At the moment I feel it won't, but I believe it will pass 10 million copies.
So Halo: Reach will outsell GT5 by at least a small margin.