This whole question depends a lot on whether you want looks or performance. Most of the springs labeled "Sport" actually lower the Mustang too far and do more harm than good to the suspension and steering geometry. Most of the guys who need serious cornering performance, i.e. autocrossers, get the bullitt kit, which is FMS C springs and matched shocks and struts.
With a small drop in ride height, you will be able to maintain proper alignment without CC plates, however our suspension is so limited without them that they're a good idea anyway. The MM double your available castor from 2 degrees to 4, which means better stability at high speeds and under braking.
Another factor, as was mentioned is bumpsteer. Lowering the car at all will change the angle of the tie rods, increasing bumpsteer, something the Mustang desn't need more of. A bumpsteer kit, or more accurately, adjustable tie rod ends, will allow you to correct the position of the tie rods, correcting any bumpsteer you added by lowering the car. Steeda sells a kit with both the adjustable tie rods ends and their X2 balls joints, another part that helps correct the a-arm geometry after lowering. I don't understand it that much, but everybody who uses it says it works well.
The other thing that was mentioned was rack bushings. Again, only a performance issue. If you're going to be ripping around a track or something, get MM off-set aluminum steering rack bushings. Otherwise you can get away without it.
So to make a long post short:
If you're going for looks, get the springs and get the CC plates if you need them for a proper alignement.
If you're going for performance, Springs, shocks and struts, cc plates, bumpsteer kit, X2 ball joints, off set steering rack bushings and if you don't already have them, subframe connectors. All the extra cornering ability will come through much better with a stiffer chassis.
For further explanation:
http://www.miracerros.com/mustang/t_suspension.htm
With a small drop in ride height, you will be able to maintain proper alignment without CC plates, however our suspension is so limited without them that they're a good idea anyway. The MM double your available castor from 2 degrees to 4, which means better stability at high speeds and under braking.
Another factor, as was mentioned is bumpsteer. Lowering the car at all will change the angle of the tie rods, increasing bumpsteer, something the Mustang desn't need more of. A bumpsteer kit, or more accurately, adjustable tie rod ends, will allow you to correct the position of the tie rods, correcting any bumpsteer you added by lowering the car. Steeda sells a kit with both the adjustable tie rods ends and their X2 balls joints, another part that helps correct the a-arm geometry after lowering. I don't understand it that much, but everybody who uses it says it works well.
The other thing that was mentioned was rack bushings. Again, only a performance issue. If you're going to be ripping around a track or something, get MM off-set aluminum steering rack bushings. Otherwise you can get away without it.
So to make a long post short:
If you're going for looks, get the springs and get the CC plates if you need them for a proper alignement.
If you're going for performance, Springs, shocks and struts, cc plates, bumpsteer kit, X2 ball joints, off set steering rack bushings and if you don't already have them, subframe connectors. All the extra cornering ability will come through much better with a stiffer chassis.
For further explanation:
http://www.miracerros.com/mustang/t_suspension.htm