Wow. Great Thread. Another Greatest hits with added content was Midnight Club, which shipped with Tokyo as an added city on the greatest hits. However, GrandTheftAuto San Andrea's shipped a collector's edition with an extra disk (after the hot coffee scandal), but didn't include any extra's in the greatest hits disk.
My guess, it will NOT ship with the additional disk. As it is a greatest hit, if they can fit more content onto the dvd (and they have more content), they will, but if the dvd is already full to the brim with the original game, they will not add the extra disk, as they have never added an additional disk to Greatest Hits. (Note: I hope i am wrong on this, as i actually do not have the game, and would pick it up new, with the extra disk, for 29.99. I have a weakness for extras, but i won't pay more than $10.00 for them.)
Regarding the quality of GameStop disks, the clerks at my local one know me, and will always show me all the copies of used games they have, so i may pick the best one. I simply only accept absolutely pristine disks, used or not, from GameStop. (Note, GameStop will often put used stickers on brand new games, if they are not selling.)