Not even boutique built, the GTM is your garage-built.
You don't see many kit supercars, but it makes sense for the company manufacturing this one to be Factory Five. Known primarily as one of the biggest manufacturers of AC/Shelby replicas, Factory Five also offers the GTM, a kit car of their own design. The majority of the mechanical components come from the C5 Corvette, including the engine, although there are some Porsche 911 parts thrown in there as well, so don't go saying it's a rebodied Vette.
Car & Driver once called it "the fastest car you'll ever build in your garage," and some examples have been built with 1600 horsepower and the ability to hit 60 in under 3 seconds. They are extremely rare, but here we see an example in the wild.