As long as you don't lower the car more than 1.5 inches bump steer doesn't really effect the 1990 and newer Mustangs.
My car is lowered 1" in front and 1" in the rear. I have subframes that hang down a little, ground effects, and a T/A. None of it scrapes unless I go into a parking lot with a big ^ at the entrance. You guys know what I'm talking about... Anyway, you just hit that at an angle and it doesn't drag. It also depends on what size tires you're going to be running. I have 245/45/16's.
You might not have to have C/C plates, but they make a big difference in handling and tire wear. I would suggest getting them.
True, the ride does get a little more harsh when you lower the car, but that is where some really good struts and shocks come into play. Bilstiens or the 5 way adj. Tokicos. My ride is firm, not harsh. Ever ride in a new Vette? That's how it feels. Forgive me for comparing my Stang to a Chevy.