Re: flowmaster american thunder 2005 v6
I had one and it sounded great outside the car, not too loud at all. But, inside the car was a different matter, especially after I put the 3.73's in. The drone was so bad and it was so loud inside on the highway that I couldn't talk with the passenger or hear the radio. That may be different on a coupe, but it was too much on my convertible with a 45 mile one way commute each day.
As far as horse power, some have said they picked up a couple with a better flowing muffler according to dyno readings, but not so much that you would really notice it from your "butt dyno"