Wes is right. ST* allows you to fix the static camber issue, stiffen up the springs to make it transition faster (and more importantly keep the dynamic camber under control), balance the car with bars and said springs, and put on some real wheels. It has many other allowances, but just getting 225 or 245 tires on 7-8" wheels and keeping them flat is 90% of the improvement. In GS you're not only outgunned more severely, you're driving a car that's not working right: to go fast, you'll be riding around on the edges of big fat tires. That's expensive, and not that fun.
