With 19 inch wheels, the brake rotors must look puny showing through. I'd consider buying a GT rotor/caliper bracket upgrade to put the larger front brake rotors from a GT onto your car. Better braking, and the rotor will "fit" better to those large 19 inch wheels. The rears are the same for our V6's and GT's so they will still look small.
I do not have a problem with the "ride quality" of the Mustang, but then again, I don't carve out twistys and curves each weekend with it. I like the straight forward ability the solid axle offers. I wish the new Mustangs offered both options to potential Mustang owners, this way those that are looking for twisty type road handling could buy the new split rear axle design, and the "go straight" drivers could stick with the solid axle.
As to the rear axle mods, there are lots of posts here that offer suggestions as to how to keep it's "triangle" effect better planted. Panhard bars are one (price is reasonable), and Watts link is another, this one being a pricey mod.
It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt.