AMJ ..A flowmaster muffler is one of the most free flowing mufflers out there. it has no insulation which will burn out and cause problems later on in life on a turbo muffler. cherry bombs are loud and that's about it. where is your proof on a flowmaster being not high flowing.
Flowmasters are baffled and chambered meaning the air is routed through several different chambers (2 in the 40 series) and around a variety of baffles (I want to say two or three per chamber in the 40 series) so how can that be free flowing? It's not as bad as your typical OEM muffler, especially the old school ones CUCVs come with, but they're definitely not as free flowing as a straight through muffler.
Yes, if you manage to get a Cherry Bomb very hot it will burn out. However at $25 each you can buy three for every Flowmaster serving a similar purpose.
Personally if I was going to spend $80-100 I'd buy a Magnaflow or Borla muffler. Stainless steel, straight through, and quality built.