Of course its a possibility, its a possibilty that it wont as well. Anyone who denies that is just being a fanboy. And it looks like they are already here in bunches.
I personally believe the GT5 will outsell Halo Reach. Oddly enough for the same reason most in here are saying it wont sell well. The userbase. I believe GT5 will sell a minimum of 10M, looking to end closer to 13M, can Halo Reach beat that? Sure it can.