I have a thing for big diameter brake kits. I have one of PPI's 13" Cobra brake kits on my '88 LX along with rear SVO/Mark 7 11.35" disk brakes. It hauls my light LX down from speed REAL fast.
http://www.mustangparts.com/suspension.htm
Most likely i'll go with one of their kits again. I might get the red Bullitt 13" kits this time and paint my rear calipers red to match. I plan on doing the fronts first. Hopefully by then maybe Baer will have come up with a solid 13" rear rotor for their kits. I am really not into the slotted/crossdrilled thing. I want a big rear disk to fill in the empty space that will be there when i get my 18" bullitts or Saleen wheels.
For $250 though, it's a steal. PPI sells the Cobra/Bullitt 12" (11.65") rear brake kits for $400 but you need to pull the axles to install these. The one downside to this rear kit is that it uses a solid 1/2" thick rotor like the GT/V6. The Cobra/Bullitt/Mach 1 uses a 1" thick vented rotor that disappates heat better. If you plan on doing any racing or anything, then this kit really isn't for you. That solid rotor will warp real fast. But for most of us street guys, it will fill in those rear wheels quite well, and i think that's what most of us are looking for.