Higher grade gas wont do anything for you unless you change the tune. Your best bet would be to go with catback exhaust and a chip or tuner (SCT makes the best for mustangs in my opinion). In addition to that you could also go with headers, gears, and better tires (drag radials). The last thing would be throttle body and underdrive pullies. That is pretty much it as far as the less expensive mods go. After all that, you are looking at a power adder like a supercharger or turbo charger and modding your engine. There is always nitrous, but I am not a fan of nitrous. I like mods that give you the extra power every time you hit the gas pedal, not ones that are only there when you press a special button. If it were my car, I would do the catback exhaust with the chip or tuner first. Then replace your rear tires with some Nitto drag radials. Nitto makes a great drag radial, it provides awsome traction and has great tread life for a drag radial. These upgrades together will run you around $1,000 and add around 20-30rwhp to your car. The addition of the drag radials though will really help your 0-60 time. With those 3 mods you can probally drop 3 tenths off your 0-60. Maybe more depending on your driving skills. After that, changing your gears will probally make the next biggest difference on your acceleration. Then do the headers followed by the throttle body and underdrive pulley. If you are really serious though, just do the exhaust, chip, and tires. Then save up around $4,000 to $5,000 and have a supercharger installed.
You need more power? No, you want more power. lol You can go on google.com and type in something along the lines of Mustang upgrades, there is a huge aftermarket for Mustangs. You should be able to see what the various companies stock and what the prices are.