Carlos already explained some of it.
But to get back onto you original question Nick, about the Turbo/Turbo S.
Think of the Turbo S not as a separate model, but a trim package. While the Turbo S is a bit higher in price, Porsche throw in a lot of stuff as standard, PCCB and such, after factoring those standard equipment the price difference is not that great anymore. Consider the extra HP just as a bonus from Porsche. This hold truth for the 911 range, the Cayenne and the Panamera. (Well for the incoming Panamera Turbo S Porsche will break from tradition and the Turbo S will be very different than the regular Turbo)
Realistically, how many GT2/GT2RS the market can absorb? 2000 cars? Or maybe less. An educated guess puts the GT2/GT2RS firmly in the price category of Ferrari and Lamborghini, both of which are much more attractive to some people with their exotic styling. Porsche really don't need to limit the production, the small target audience will take care of the small planned production volume anyway. Granted, the North American 918 VIPs will all take a car, but that's less than 300 total anyway.
To earn the maximum amount of profit per car, Porsche would naturally produce the highest margin car. With the exotic material roof and fenders developed for the GT3RS, one would naturally expect the GT2RS to have the carbon fenders and the magnesium roof from the GT3RS, Would Porsche cheap out and go back to steel on the GT2 just so they can 'price' the cars correctly and create more profit margin?
Admittedly, I do not have a line to the insider information on the car, I am just trying use some common sense and business logic to come to my conclusion.
--