Robbie.... if looking for not only a lower ride, but an improvement in ride quality, I would suggest a set of properly matched dampers to go along with your new springs. You are best to also install a set of CC plates, to address the negative camber you will gain on your drop. This will also allow you to add a bit of caster, to improve your grip when cornering.
You might contact Sharad of UPR or any of the several site sponsors here and will be happy to give you some input on what may be the best way to go. Keep in mind, that suspension mods add NVH. This is a by-product of nearly all suspension mods for the Mustang, so consider this, if you begin to build your suspension up.