Sony isn’t well known for keeping secrets and Gran Turismo 5 XL has been poor at best. After it’s original appearance as a rewards prize, the game started to appear on retailer sites for preorder. Not much more information is known past it including over 20 dollars of DLC. This strongly supports the theories of it being simply Spec II. In either case you can find the box art below.
If you’re interested in buying, retailers list this as a January 17th release.