Can anyone tell me what the difference is between the Mustang Roush and The GT??  

There is a big difference between the two depending on what you send it into Roush for. They do everything from the interiors, to bolt on's, other upgrades to the motor, upgraded exhaust. Different front ends, side skirts, rear ends, louvers, rear spoiler. So there is a big difference from the two. I have a 2005 Roush Mustang and before I sent it in. I liked driving my mustang. But after I sent it into Roush I never want to get out of it. I don't know if that is what is what you were asking. If you want to know more go to this website>  http://www.roushperf.com/ That should help you out.