It will reach it
Gran Turismo has no serious competition but games that aren't mindless shooters have it hard in this Gen.
The taste of gamers changed a lot - Kids in the last Gen wanted cars but now they want guns.
So its far more remarkable Gran Turismo 5 managed to sell that much...
You have to think - This game did cost more than 50 or 60 Million and had the money alone through the prologue version nearly completely back.
If we would still have the same gamers from the PS2 Era GT5 would have sold with ease 15 Million or more units.
Maybe people get tired of the copy paste shooters in the next years and will push Gran Turismo 6 to the numbers it deserves.