I got the Steeda UDPs on my 98 GT. Can't say I had a big improvement with them, but certainly no loss (obviously). But it does rev up WAY quicker, which is a plus. And there's no possible way fuel mileage could go down, hopefully should go up but I can't say for sure yet.
I have NO cooling problems and charging problems are not anything to worry about. In my car, with the lights, AC, and 1000 watt stereo, with my foot on the brake in drive, the volt gauge drops off a little. Soon as I touch the gas it goes right back up. Needless to say, I haven't had ANY problems with them. I would recommend them.
-Will