While still more restrictive than an after-market exhaust, the 99-00 models are much less restrictive than 96-98 models. A cat-back system or just mufflers won't give you any performance increase, although it might sound a bit meaner..
If you're serious about performance improvements out of your exhaust, an X/H pipe is really your only serious choice.
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98 GT
Dyno-Max cat-back exhaust, 3.73 gears, Steeda shifter, Ford Motorsport Underdrive pullies, 87mm MAF and Roush cold air kit...