Tony had mentioned market share. You have to consider 15" wheels are 1960's technology. I imagine, tire companies do not recognize market share in lesser performing tires. Shorter sidewalls are essential for handling. Currently, the best handling tire and wheel packages are on 19" and 20" wheels, the latter to accommodate larger brakes. Z/28s and GT350 were tested on multiple tire and wheel packages and 19's had the best grip. CSX 3170 runs 19x10.5 and 19x12 and we know Bruce Cambern is 100% about handling.
Perhaps you consider keeping the 15s for shows and daily driving and get some 17, 18, or 19s for the track. The 15s with 2020 technology will not rival the larger wheel packages.
I wanted the 15's for my car, but opted to go with 17's to accommodate bigger brakes. The reality is when it comes to handling, 15's have gone the way of the Dodo.
https://www.fuelcurve.com/twice-bitt...-shelby-cobra/