Halo Reach is not going to have any problems outselling Gran Turismo 5 and I will tell you all why. If you look at the history of the Gran Turismo series Europe has always been the biggest market for the game but America has always been the second largest.
With each iteration of the Gran Turismo series the Americas have always sold 2x as many copies of Gran Turismo when compared to Japan.
This means that they would need America to show the game a lot of support for it to have a shot at outselling Halo Reach which isn't going to happen. Because this gen the fps is king in America and so is COD and Halo. And no Cod is not going to hurt Halo because each time a main entry in the halo series has released in the same year as a Cod title Halo has outsold that particular Cod title.
Edit) Just some further comments
If you look at the past and current Grand Theft Auto has always been just as popular or more than Gran Turismo in Europe so we can probably look at how many people bought GTA 4 to see what optimal sales would be like in Europe. It's currently #2 in sales with 3.43 million sold in Europe according to vgchartz. MW 2 is in front of it with 3.71.
The jrpg has always sold best in the past and currently in Japan. The bestselling title on the ps3 in Japan is Final Fantasy 13 with 1.85 million sold.
So lets say that GT 5 sales 2million in japan (only GT 1 has sold more than that), 4 million in Europe(making it the bestselling ps3 title in Europe to date), and 2.7 million in the Americas( putting it slightly behind GTA 4 since GTA has always been way bigger than GT in America)
That equals 8.7 million ltd sales.
Let's say Halo Reach does 7million in the Americas (placing it between Halo 2 and 3 sales) and 2 million in Europe (once again placing it in between Halo 2 and 3). That's 9 million in sales and it doesn't have to break any records to do it.
Since Halo Reach is being bundled and also the fact that this is Bungies last game I expect sales to be way higher than 9 million.
