This was taken from DF. Credit to HammerZ71
The following is a compilation of some popular mufflers and their CFM rating. Basically, how good they "flow"...
Dodge Ram OEM: 285-355 depending on year and engine.
Walker PF29: 217
Thrush CVX: 260
Thrush Boss Turbo: 297
Gibson Superflow: 311
Hooker Super Competition: 320
Summit Turbo: 331
Flowtech Afterburner: 342
Summit Fully Welded: 343
Flowmaster 40: 352
Flowmaster 50: 362
Dynomax Super Turbo: 410
Cherry Bomb Pro: 470
Cherry Bomb Extreme: 500
Thrush Magnum Glasspack: 507
Cherry Bomb Glasspack: 510
Edelbrock RPM: 579
Cherry Bomb Vortex: 580
Edelbrock 304: 640
Dynomax Race Magnum: 711
Magnaflow 12388: 767
Borla XR-1: 836
Dynomax Ultra Flow SS: 1000
It is important to note that without some engine modifications, too much flow can cause damage and actual decrease performance. It is generally accepted that the maximum rate you should ever try to achieve is 600 cfm on a stock to mildly modded truck, which is the rate air will flow thru a straight 3" diameter pipe.
I was unable to find any data on Magnaflows, if any body has some reliable data on them or any others not listed, PM them to me and I'll add them...
Some sound level comparisons I've been able to find:
1.Borla XS, 80db at idle, 90db at 2,000 rpm, 110db at WOT.
2.Hooker Maxflow, 80db at idle, 90db at 2,000 rpm, 120db at WOT.
3.Magnaflow 14", 77db at idle, 86 at 2,000 rpm, 104db at WOT.
4.Flowmaster 40 series, 82db at idle, 93 at 2,000 rpm, 115db at WOT.
5.Flowtech Warlock, 83db at idle, 92 at 2,000 rpm, 112db at WOT.
6.Dynomax UltraFlo, 83db at idle, 94 at 2,000 rpm, 113db at WOT.
7.Borla XR1, 83db at idle, 96db at 2,000 rpm, 118db at WOT.
8.Bassani Street, 83db at idle, 96db at 2,000 rpm, 120db at WOT.
9.Edelbrock RPM, 84db at idle, 93db at 2,000 rpm, 118db at WOT.
10.Bassani Real Street, 84db at idle, 96db at 2,000 rpm, MAX. (120+db).
11.Flowtech Afterburner, 86db at idle, 94db at 2,000 rpm, 115db at WOT.
12.Flowtech Terminator, 86db at idle, 94db at 2,000 rpm, 119db at WOT.
13.Hooker Aerochamber, 87db at idle, 94db at 2,000 rpm, 114db at WOT.
14.SpinTech, 87db at idle, 97db at 2,000 rpm, 116db at WOT.
15.MAC, 87db at idle, 98db at 2,000 rpm, 119db at WOT.
16.No Muffler, 91db at idle, 103db at 2,000 rpm, MAX. (120+db).