My Flowsound (Flowmaster knock off) Muffler on my 4.7.
Interior Sound at Idle: Low, rumble
Exterior Sound at Idle: Low Rumble
Interior Sound at Freeway Speeds: 45 ish is a bit loud. Real deep. After overdrive kicks in its manageable. Havent had it up past 70, but im thinking about 75 she will get loud.
Exterior Sound at Freeway speeds: Not sure
Any performance gain advertised vs felt: Not really
Size of muffler vs the Stock setup: just a replacement, I think its a little more tucked under there than the big old stock one.
Overall impression: loud and I love it!! best 80 bucks I spent.