I also own a GT. 2014, Sterling Grey, black leather. Weird.
Boost it. I've got a Roush Phase 3 Kit for sale if you're interested. Made 600/540 on 8.4 psi. No built motor needed.
http://www.roushperformance.com/parts/2011-2014-Ford-Mustang-GT-Supercharger-ROUSH-Phase-3-675.html
Only has 400 miles on the setup.
Other than that, suspension is the way to go. I've got a full BMR setup going on here in the next few weeks