Categorizing and pigeonholing the various car types used to be a lot easier back in the day.
Something with no top or soft-topped that was drafty, bare and uncomfortable but unusually quick and wieldy for its size was a safe bet as a sports car, while GTs were a high-style device with luxury appointments and comfortable demeanor that also had the ability to cruise rapidly and effortlessly long-distance, even if small-engined.
Then you get to consider conundrums such as the Jaguar XK120 which was either a fast but unreliable sports car, especially in roadster trim, or when presented as a coupe became a fully fledged GT that still broke down a lot!
So which is what? I guess its the same dilemma with Corvettes, reliability notwithstanding. When in individual perception does the original Lotus Elan turn from sports into GT .. when it's the hardtop, or the Elan 2+2?
Or to use your 599 analogy; if Ferrari presented a version with an ill-fitting single-layer soft top, would that one modification transmogrify an otherwise GT-style machine into a reasonably quantifiable sports car - and vice versa? Could the hypothetical difference between sports/GT hinge on a basic soft top vs insulated & triple-layer? The question becomes 'how many times can you split a hair?'
Q-cars are easier, though. The objective is covert invisibility melded with unusual speed, which as my favorite combination is what I voted for
(lots of responses since i started replying!)