The m80's help smooth out the sound and get rid of the backflow kind of fart sound between shifts or when decelerating if you know what I mean. They do quiet it down a but but make it a lot smoother than a straight o/r x.
My car came with an o/r x and Magnaflows and it literally sounded like it was farting between each shift. Put on the m80 mid mufflers and it totally alleviated the problem but they made the exhaust pretty quiet when you weren't getting on it. I then had the Pype Bombs which were very similar to the SLP's and it sounded pretty awesome. Hardly any rasp but didn't have a ton of tone and they droned a lot. I got a ton of compliments on that set up but I drive on the highway a ton and the drone got to me after a few months. If my Mustang wasn't a daily driver I'd still have the Pype Bombs.
I now have the violators with the m80's and love the sound, just wish it was a little bit louder but it still has a real nice rumble.
Torque: It's like crack for your right foot