The GT is definitely designed
The GT is definitely designed to be a Track car out of the box. The only problem is, as I understand it, if you use the Front brake locks to warm up your tires, the warrantee goes out the window.
The IRS may be working against it a mite in a straight line, typically a solid rear axle is the better set up for going straight. The gains in handling are probably well worth it, especially in everyday driving. So it's not really surprising that the new gen. has about the same 1/4 times. I imagine, on a road course, the 2015 having the independent rear set-up would be significantly faster.