Brother had a friend who had a Camaro with Flowmasters on it and the car picked up several tenths once he removed them. People buy those based on what they sound like. Nobody I'm aware of that has a dedicated race car uses those things. They are very popular with Mustang and Camaro guys for some reason.

Hooker Aero Chambers are good.

I don't like mufflers with packing material in them. They end up getting louder over time. Motorcycles come that way and you have to rebuild the mufflers every so often. Can't do that with a sealed car muffler.

I don't believe muffler flow bench numbers and I don't believe cylinder head flow bench numbers. Some made up machine cannot duplicate the airflow capacity of a running engine.