Actually, given this thread is about two games that haven't released yet, the statement "Is is possible for GT5 to outsell Halo Reach LTD" is essentially unarguable. Of course, statistically it's possible - that can't be argued with.
The real question implied in the OP is what is the probability it will?
I'll give it 67% chance with 33% chance Halo Reach outsells GT5 LTD.
What odds do you give?
Try to be reasonable... its easier than you think...