If you don't want to sacrifice ride quality, your best bet is either the Bullitt/Mach 1 Springs or the Vogtlands along with a good quality shock/strut combination. I had Vogtland springs on 2 of my other cars, although neither was a Mustang, the ride quality was excellent on both of them, so I went with them for my Mustang also, along with the Koni STR.T shocks and struts.

The reviews on the Vogtland springs all say the ride is very close to factory, but with improved handling.