I just "hollowed" out a very large ,single exhaust, Cat on my truck.
It DEFINITELY made it louder and it does seem to be more free flowing, which is understandable if you've ever seen the "Honeycomb" material that's inside of one... HOLY COW!!!... It looks restrictive.
The exhaust sounds a bit "twangy", because now you can hear the explosions in that big empty chamber. Like Soccerluvr said, a pipe welded inside would solve that and make a better flowing exhaust system in the process.
I don't think that is the answer on our Mustangs though. Especially if you need it to pass smog. You can buy an Off Road pipe for relatively cheap and still have your intact original exhaust to put back on when it comes time for smog inspection.
It seems like most of the guys on here do that and it's the smart thing to do if you live in a "smog nazi" state like I do.
The reasons that I gutted the one on my truck are; It was already broken and rattling so I thought, what the hell.
And I'm doing it as an experiment to see if the truck passes smog with it gutted.