A lot of my friends had 87-93 Mustang GT's running anywhere from mid 11's to mid 14's. One of the things that slowed the cars down was the the deep rear bumper. It acted like a parachute. Thats why if you notice the "Cobra" version (more performance) of the same years has a totally different rear bumper cover and different side skirts.
GT
http://memimage.cardomain.com/ride_i...0113_large.jpg
Cobra
http://www.dragtimes.com/images/9094...rd-Mustang.jpg
BTW the rear wing on the Cobra was totally terrible. For High Speed runs people would tape them shut