How much does it cost to supercharge a 2005 Mustang GT?
If you want the finest supercharger on the street that money can buy, then you need to seriously consider Whipple. Plan on spending about $5K. They're instant boost. Because they're a screw blower, they do not heat the intake charge. Since they don't heat the intake charge, your boost to octane threshold goes up tremendously. Step down on your spark plug heat range at least one step. Try the Denso iridium power and don't change the gap on them out of the box.
If you need help in selecting your spark plug heat range, I'll do that for you.