no flowmaster, flowmaster bad.
Hooker Aerochambers are nice and quiet for aftermarket chambered mufflers. No drone at speed, but they're noticable at throttle punches.
Dynomax ultra flows are nice mufflers, i have one on the truck, i like their straight through design and lack of drone at highway speeds, compared to the single bullet i had on my LS1.
Summit racing sells a Dynomax ultra flow series muffler that is a 2 in 2 out 3" unit with an internal X pipe. This might be a good idea for the GTO where the two pipes come close together.
Magnaflow 4x11's or similar single in/out mufflers are also good designs, not extremely loud, and are good choices for true dual exhausts with no cats and longtubes. A little more expensive than the above ones though..
I would suggest a removal of the resonators, installation of an X pipe, and replacement of the stock muffler/mufflers with one of the aforementioned mufflers.
Hope that helps you dude, hopefully you'll get your own GTO one of these days.
EDIT: I should mention that as far as i know, none of these manufacterers (accept magnaflow) make a catback specifically for the GTO. I've always had custom exhausts put together, just because it's way, way cheaper. If your parents would rather install their own system and not mess with a bunch of custom bend and welded parts, that's cool.