Cat back systems are good for sound, that's about it. If you want to see a performance increase, you would want to pair it up with a good X/H-pipe.
As far as sound goes, my favorites are Dynomax and Borla, and Borla is just plain expensive...
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1998 Mustang GT
14.18@100.56
"Traction Impaired" or "Nitto Needy"