Flowmasters won't yield as good of results as a Magnaflow or another company that uses a straight through design muffler.

 

They are stupid restrictive, just by design. Depending on the pipe size and muffler position, they sometimes sound good, but for an aftermarket muffler, there's much better choices out there. my personal favorite is the hooker aerochambers, awesome flow, great sound.

Just for an example, some of the turbo v6 cars out there struggle to build boost with the flowmasters, just opening a cutout was yielding HUGE spool time improvements, and the max boost they could hit in general. Problem was solved by using a better flowing muffler eventually, one example of many.