The lowest springs for our cars, are TEIN S-TECH. The lower the car 2.5 inches in the front, and 2.3 inches in the rear. These are the springs that I currently have on my car (2003 Mustang GT). The ride quality is "GREAT" (however you do feel every bump in the road, and from time to time have clearance issues). I speak from experience, as I have also had eibach sportlines and h & r supersports. The TEIN's are far superior to the previous mentioned springs. No one I know has these springs, so there is very little info about them. WARNING: IF YOU DO GET THESE SPRINGS, DO NOT BUY OFF EBAY, THOSE ARE COUNTERFIET!!!. Go to TEIN's website and click your state for a local vendor. If they quote you more than 175 for the set you are paying too much. Also, my current suspension setup is...
1. TEIN S-TECHS.
2. Strange Adjstables.
3. Steeda bumpsteer kit.
4. Steeda ball joints.
5. Kenny Brown caster camber plates.
6. Maxim Motorsports panhard bar/brace.
7. Steeda subframe connectors.
8. 18*9 with 265/35/18 up front, 18*10.5 with 305/35/18 in rear (DD Bullits).
9. Maxim Motorsports wheel spacers (cant remember size).
10. VERY IMPORTANT: ROLLED FENDERS!!!
All this and no tire rubbing/cuts. However I do not recomend this suspension to everyone. My car in no way rides like a Cadilac! Its like jumping in a race car (without that much power

), and driving to work everyday. It takes a little skill and knowing the roads, so you dont kill your car.
P.S. If you get these springs. USE THE ISO'S THAT COME IN THE KIT. NOT STOCK OR OTHER AFTERMARKET ONES.