I have a 2K GT convertible. Living in E.TN, my little playground are the mountain roads. I stiffened mine up for that purpose with the following suspension pieces: SR front and rear shock tower braces, SR front and rear sway bars, Maximum Motorsports rear upper (rubber bushings at frame and rear axle housing) and HD lowers (poly at frame and spherical at rear axle housing), Southside Machine bolt on subframe connectors, and Koni STR.T struts/shocks. The sway bar change made the most noticeable change in ride and feel, but the other pieces combined make it feel like a big go kart.
Recommend reading Maximum Motorsports website concerning Mustang rear bushings...believe the rubbers in the uppers are the way to go...poly or spherical in the lowers. The Konis are a nice addition and they can be bought from any of the Mustang suppliers or somewhere like Shock Warehouse (I bought mine there)...Shock Warehouse has specials on Koni from time to time. Other pieces at your discretion.
Btw, if you are in E.TN, I have the GT sway bars (fr/rr) I took off...some of the V6 models do not have a rear bar and these are an easy upgrade/install. Free to a good home...I don't want to ship them.