Exhaust Setup Questions

Ok my friend just gave me the a set of Mac Longtube headers off his car... I think I wanna get a Mac Prochamber and Flowmaster's or DynaMax mufflers with turndowns right before the rear axle... what kinda performance increase will I most likely see??? I have 2 cats, stock H-Pipe and DynaMax bullit mufflers on there now... so I wouldn't be running any Cats with the new setup and I'de get it tuned by a Mustang Shop down here to get all the 02 sensors off...

Later
Jake