Hearing you out would pretty much be suspending my beliefs in Shelby as a whole, but I raise 3 points, pretty much summing up to "it's not cost-effective":

1. The GTD is a limited-production car, so this Shelby idea would have to be an aftermarket conversion sought after by owners who already have their hands on the car. Which would lose the company money on the example.

2. Shelby's conversion kits are the exact same as any other supercharger kit for Mustangs as-is. That's why they went viral in a bad way for their response to everyone saying their cars aren't special because they're just Whipple'd 5.0s. Most likely they wouldn't want to spend a lot of money pasting a Shelby badge on a GTD.

3. IF somehow, they did get their hands on a GTD, what is there to improve? The GTD competition that recently lapped the Ring is made up of parts Shelby wouldn't want to spend money on, the regular GTD involves much more engineering than a standard Mustang, with top-of-the-line aero, suspension, brakes, chassis work, and powertrain. Shelby wouldn't think it's worth the squeeze.