I had a two and half inch system on my old 440 powered BB Dart, Hooker fender well headers into the 2.5 inch pipe back to a set of original Thrust Corvair turbo muff and then 2.5 pipes back to the front edge of the rear tires for a side exit.
I ran the car three times at the old LACR track, it ran a best of 12.23 ET at 109.+ MPH.
I removed the exhaust system completely, not very heavy either and it ran 11.39 ET at 117.+ MPH with no other changes
My Duster had a full three inch system with Magnaflow straight through 14x7 side entrance center exit oval muffs mounted behind the rear end at the rear bumper, it picked up .02 ET and.2 MPH in the 1/8 mile by removing it with no other changes. That system was a little heavier than the Dart was so maybe it was the weight that made the Duster faster and quicker
The Duster was a lot louder than the Dart at WOT
I never got stopped or warned about exhaust noise on either car by the local law enforcement