I recently added the Steeda lowering springs to my '06 GT. I think they said 1" on the front and 1.2" on the rear. Personally, I love the look of a lowered 'Stang in this body style; the off-balanced stock appearance is unattractive and offers no benefit, imho. However, that said, and even though mine isn't a daily driver I can't imagine having gone lower due to the stiffness of the ride. It's like my own personal little go-kart. Too rough for a daily driver, but for the curves and over all handling I'm happy.
Did you just do the springs? Or did you put in new links/brackets too?