Magnaflow Magnapacks or Borla Stingers. I personally like the Stingers just a bit better.
Magnaflow Magnapacks Catback with Mac O/R H pipe - YouTube
2003 Mustang Gt Borla Stinger Bassani O/R X - YouTube
Not too loud at part throttle, but it gets REALLY loud when you get on it. What midpipe and headers will you be running?
Also, about the loss in low end torque. It is of my personal opinion that that is complete BS. I've heard it SO much on so many different forums, and I've never once seen any proof to back it up. My personal experience is a .2 second and nearly 2 mph gain in the 1/4 mile with an offroad midpipe and catback. And just because I'm bored right now, I'll go into a more in-depth explanation.
Back pressure, which is simply the force against the air flow created by catalytic converters, restrictive mufflers, bad bends in the pipe, etc., is ALWAYS a bad thing. Yes, your car requires a certain amount to do some complicated stuff inside the heads, but that amount is easily provided by the friction losses and aerodynamic drag created from simply moving the air from the front of the car to the rear via the exhaust pipes. The cats create quite a bit of back pressure, and removing the cats (and/or restrictive muffler) will decrease that pressure and create more power.
The problem most people have is separating back pressure from exhaust velocity. Faster moving air helps pull the exhaust gases out of the combustion chamber better (read: scavenging), which is the reason why long tube headers work. Your factory exhaust has a 2.25 inch diameter, from the exhaust manifolds back. If you replace that with a 3 inch exhaust, the speed of the air moving through those pipes is going to decrease (as per some relatively simply fluid dynamics principles), although not by a whole lot because the 2.25 inch piping itself isn't really restricting much in the first place. So, slower moving exhaust = less torque.
When you go and put an offroad midpipe and/or catback on your Mustang, you are both removing back pressure (by removing the cats, better flowing pipe bends, etc.) and decreasing the exhaust velocity (by increasing the diameter of the pipe). However, the amount of power/torque you gain by removing the back pressure is MUCH greater than the amount of torque you lose from the decreased exhaust velocities.
Make sense?
This post should be a sticky. I get tired of explaining the difference between back pressure and poor exhaust scavenging. With any exhaust system, there is a BALANCE. That's why there exists such a thing as a Tuned Exhaust System and also why I chuckle when I see exhaust manufacturers talk about how their products will increase HP with no dyno data to back it up.