I generally agree with STUNTS. Since I believe he is talking about a combination of desirability PLUS affordability. There are simply less buyers at the top of any market. It doesn't mean that most of the enthusiasts wouldn't want it, but not everyone can afford it. Plus, some people are simply not aware of the Ford GT. Think about how fewer people are aware of the GTX-1.
If you were to ask Mustang GT coupe owner, would they take a convertible Ford GT500 at the same price, a vast majority would. They would see the additional value and worth. Sure, some would still want a coupe GT, just as some would still want a factory Ford GT, over a GTX-1. Obviously, a GTX-1 is a much greater and complex conversion than a factory Mustang GT vs. GT500, but this is for illustration purposes only. If you want, swap Ford GT for GT500, and know that some people would still want (and be able to afford) the base Mustang GT.
If you consider that in today's market, a GTX-1 likely costs more than a Ford GT, it has a higher entry point price to uphold as well as less people that can afford it now, and in the future. Assuming equality, the same percentage decline, will lead to a greater reduction in dollars.
All speculation of course, I definitely don't know what will happen.
The important thing is to buy the one that you want and admire and enjoy it. If you are buying something to please a future potential buyer, what's the point? Unless, it is your livelihood...