I could be wrong but I'm pretty sure that they are the same as Borla Stingers of Ford Racing mufflers. Search one of the above and you should get a pretty good idea of how they sound.
Here's the info I have on it, copied from "The Ultimate Sticky"
MBRP exhaust:
MBRP produces 2 axle back systems for the s197 Mustang GT: One is similar to borla in almost every way, the other is a muffler delete that basically replaces the mufflers with a section of straight piping. It is loud, it will be a little raspy, but it will be very deep. Very similar to the SLP Loudmouths. Works better with a catted x pipe, can also be paired with an offroad h pipe but will be obnoxiously loud.
The Mufflered version tones the car down much more with a Borla. IF YOU ARE LOOKING FOR A SLEEPER EXHAUST, HERE IT IS. even with an offroad x pipe, it is still very quiet compared to most setups, and will flow extremely well.
I heard Joes car at the track saturday and liked the way it sounds. I can get these used for a good price. If the price wasn't so good I would just wait and get the Steeda catback. Hopefully they will sound close to the Steeda ones.