Yeah, so many people think bigger is better when it comes to brakes and it's over kill.
If you can lock up your tires, the issue isn't brakes - it's traction. Look at your tires and suspension.
An pure track SN95 will need the 13" with larger calipers. Some even 14"....but it's rare. 99% of the street driven Mustangs will never need more than the 99-04 calipers, solid rotors, good pads/fluid and SS lines.