I had a flowmaster 80 series on my 99 Grand am GT and switched to the SLP cat back and noticed more with the SLP than my flow. I think the only reason was the full 2.5 pipe from the cat back, not the muffler.

 

I think a lot depends on the type of muffler as well as the brand. I would guess that a Magnaflow 14" will probably flow better than a Flowmaster 70, and a Flowmaster 40 is likely better than a 22" Magnaflow. When I went from an FM50 to an FM70 on my truck, there was a noticable loss of high end power. Will probably be getting a Magnaflow soon. BTW I do have duals on my truck. They route the pipes together from the muffler to just before the spare tire, where they separate. Of course, my spare is in the back now.

GMC_DUDE

 

I think I may go with the magnaflow 6" x 14" & may tru duals turned down right before the axles. Then again I may just put one & leave the factory tailpipe set up.