Oh, yeah. This might be relevant.
For the time GT3 sold most of its sales, Ps2 had these userbases in the different areas.
Others: 3.1M to 16.7M
Americas: 3.4M to 21.5M
Japan: 5.6M to 13.4M
GT3 sold this in that period:
Others: ~ 5M
Americas: ~ 6.5M
Japan: ~ 1.9M
So, with this, I would say this.
GT5:
Others: 5-7M
Americas: 4-6M
Japan: ~ 1M
So I see it as a definate 10M seller.
http://www.vgchartz.com/games/userreviewdisp.php?id=261
That is VGChartz LONGEST review. And it's NOT Cute Kitten DS