What is a "base swing"?
Also, you want more air going over the top of your car, so any lower is better not only for COG but for aero. I am always leary of a company who runs a stiffer spring in the rear. You want the rear end to squat when you get on the power, and a stiffer spring does the exact opposite, along with a bigger bar. I will add, there is a reason Ford Performance, with its massive R&D budget, went softer on the FP rear springs (~30%), and a bunch stiffer in the front, along with a bigger bar.
I have been working with an IMSA suspension guy, and he has provided a lot of useful info. If you are really interested in racing/tracking your GT350, go over to M6G, or TMO. The guys there are a wealth of information. Lots of folks doing lots of different things to their cars and reporting their results.
Be warned. You will find out the FPC is not meant for hardcore race duty. Lots of discussions of cracked cranks, including the guys who cracked them. But the positive, if you don't increase your NVH, the problem is almost non-existent.
Good luck and enjoy the track time!