I don't drive my Mustang too much, but I will give my input anyways.
I have a 2001 Mustang GT 4r70w automatic. I have 4.30 gears in there also.
Honestly, I love how the gears really allow you to accelerate, and I wouldn't have anything else. Sure, the highway rpms are a bit high, especially whenever you get above 70mph, but still tolerable. As for gas mileage, I lost around 1 mpg in town with them (if that much), and I lost probably around 3mpg on the open highway. Know what else ? I honestly don't care about that little loss of gas mileage, for as I stipulated, I don't drive my car that terribly much, and I enjoy the extra performance and acceleration the gears give me.
As the other guys in this thread said, don't let gas mileage determine what kind of gears you chose unless if you frequently see the interstate, and open highways. Chose gears for performance, not mileage. If you want good gas mileage, sell your car, and get a focus, or some other 4 cylinder car. If you want performance, don't fear the gear. Do a search, and read all of the other 1 million threads on the Mustang boards regarding gear selections.