Are you looking for an actual performance increase or just a change in exhaust sound?
If you are looking for a performance increase via the exhaust system, an X/H pipe with cats is a better investment. The stock cat-back system on 99-00 GTs are not very restrictive, and you'll see minimal gains (if any), by going to an aftermarket cat-back system alone....
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"The Yellowjacket"
1998 Mustang GT
13.97@103
"Traction Impaired" or "Nitto Needy"
"They think they can make fuel from horse manure.... Now, I don't know if your car will be able to get 30 miles to the gallon, but it's sure gonna put a stop to siphoning."
- Billie Holliday