Yeah, I would say you'll likely want to add a small sub of some sort - that's a big interior to fill with sound. We just picked up our '12GT over the weekend. It came with the Bose system. I'm not a fan of anything Bose, home or mobile. The system sounds decent, although I would have been disappointed in it had I paid new car price for it. I'm an old car audio guy from back in the 90s, so I'm pretty hard to please when it comes to mobile tunes. I'd be curious to find out how the Bose system is setup - a simple speaker swap could likely do wonders for it.
EDIT: I've done some poking around, and outside of speaker sizes and locations, I haven't been able to find much regarding the actual Bose setup. It appears that the front dash and rear D pillar speakers are 3.5", while the front doors are an 8" midbass. The rear doors have a 6.5" midbass best I can figure, along with the small sub in the spare tire well. A nice 6x8 can be made work in the doors with a little fab work for a mounting solution, or can be replaced with a dedicated midbass. I found a pretty good write-up on another forum, but it was a couple of years old and the links to the pictures were gone. I haven't found any details as far as power or other configurations yet.