Is there an advantage to stainless or anodized? Will these changes help it breathe and sound better? Will I gain fuel mileage-so long as I control my heavy foot-and power? Will any of this affect the computer? VincentVia the Internet
All the various basic bolt-ons you mention are proven ways to extract extra power from an otherwise stock Mustang. Cold-air kits and exhaust are the first of the basics. A good combination of the two should get you about 20 rwhp, enough that you'll actually be able to feel the improvement. A 211/42-inch exhaust system is an excellent overall choice, and it's big enough if you decide to install a supercharger at a later time.
With an automatic transmission, your car should net about 20-22 mpg on the highway. That's not as good as a 4.6 with a manual transmission, but reasonable nonetheless. Don't expect big fuel-economy gains, but it won't hurt either. As you say, it's mostly how you drive the car that determines fuel economy. For example, I had a supercharger on my '01 GT a few years ago, and the car still got 25 mpg on the highway since it wasn't under boost cruising down the road.
If you don't like the chrome cold-air kits, there are still plenty of them that offer improvements without the bling. As for the exhaust, if you retain the stock H-pipe, then most systems shouldn't be too much louder than stock, meaning you'll still be able to have normal conversation inside the car as long as you're not hard on the pedal.