What springs do you have? At your drops I'd certainly recommend the correction kit. I'd also recommend bringing the rear up about 0.25-0.5" to get that geometry improved.
I think when Ford has offered slightly lower ride heights for their cornering-oriented Mustangs, it's never been by more than about 20 mm. Seems like as good a guideline as any.
What dampers do you have? Since you're lowered you're already out of FS class, and into CAMC. The progressives are a nice street spring but not ideal for an autocross or even dual purpose car. Regardless, you should have a better damper set than stock, something like the steeda or Koni adjustables or Ford Performance Track dampers. Ideally you want the car around a 0.5 to 0.75" drop with slightly more front if you have a small rake. This is the nice sweet spot where the geometry is still in the good range but you get the benefits of a lower cg and some free static camber (you still need more in the front though). If you want to talk suspension details I'd be happy to, and I can share with you what I've learned the past few years.
I wouldn't worry about it too much. With a little bit of work and the knowledge of the guys around here, you'll get it dialed in good! The balance should be easy enough to dial in with bars, shock settings and alignment settings. A lot of knowledge on this forum that I've used to my advantage to make my car handle great.
