This is an experiment i did. I have Magnaflow 'street series' axlebacks. They got a little louder with the long tubes and catted x-pipe, so i started wondering how the factory mufflers would sound. I reinstalled the factory mufflers just to see. They sounded good but i lost a lot of low end torque. I put my 'street series' back on. A few months later i bought some Magnaflow 'competition' axle backs, they are 'muffler deletes' (straight pipe). I installed them and i lost low end torque. I reinstalled my 'street series' and my low end torque came back.
I found the fine line between too much back pressure and not enough.
Factory muffler=too much back pressure
Muffler delete=not enough back pressure
Magnaflow 'street series'=Perfect
I'm not a Magnaflow fanboy. This is just how my testing came out with what i had to work with....