Do not waste your money on a catback system unless you want a different exhaust sound. Gains will be minimal. Go for the x- or h-pipe with or without cats for performance. Otherwise, just weld in new mufflers. Also, you mentioned that you removed the air silencer or boot. I've read that this actually caused a loss or 1-2 hp in the 99' & up stangs. The 99-01s breathe pretty well except for cats so I would go with the x- or h-pipe.
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