I have an 08 stock GT convertible. I love the car but feel it is a bit sloppy on the curves and onramps. What could be done to tighten the steering and improve the suspension? I would like to know my options.
There are a ton of aftermarket suspensions for sale (Roush, Ford Racing, Saleen, Steeda, BMR...) find one that looks like something you would be interested in. I have the Roush kit and I am happy with it. If you have 17" wheels, then getting a set of 18" wheels with lower profile tires would tighten it up a little too. Convertibles have a lot more body flex than coupes so a strut tower brace could help too. If you don't have a lot of money to spend you could just do springs to start to lower your center of gravity.
Certainly lower the car because lowering the centre of gravity will cut the roll and lowering springs are usually a bit stiffer. Springs like Roush retain some height and comfort, others are more extreme.
Firmer dampers will also assist in reducing roll and also stop the rising and falling fore and aft the Mustang is prone to.
A strut tower brace will tighten the front end but I would recommend squaring it off with something like the BMR lower A-arm support.
Finally, low profile tires will make a huge difference to the flex you feel on stock sidewalls. I use 275/35 aspect tires and it sharpens up the feel.
By using Tokico D-Spec dampers set quite stiffly, I've managed to avoid having to uprate the sway bars and the car remains comfortable and pliant.
the absolute best upgrade is shocks; Tokico D-specs or Koni Yellow. The koni's on soft settings actually ride better *and* handle better than stock.
The convertables should come stock with the lower a-arm brace. A strut tower help with the ride quality over bumps, especially on the convertables. The simplest strut tower can do; find one you like & get it.