[blue00gt]

A supercharger is probably the best mod for 99+ GT's for the hp increase for the money (as opposed to bolt-ons). On an otherwise bone stock motor most base level (mild boost) kits make about 400hp at the crank for a centrifugal supercharger and about 360hp at the crank for a roots blower. The power keeps going up from there with ntercooling and more boost.
Basically, I was looking at the same decision of whether to get a '99 Cobra or a '00 GT. There was over $100 difference per month in insurance for me, so that sort of decided it for me. Once I get the rest of the suspension and a blower I will have $32-34K total into my GT. I guarantee it will be much faster and handle better than a stock Cobra for the same money. Of course now that you can sometimes find good deals on used '99+ Cobras there may not be as much of a difference in total investment between fixing up a GT and buying a Cobra.