I recently went through something that hints that Porches are likely to be included:
Everybody knows GT includes a single page of information dedicated to each and every car in the showroom ( I'm talking about when you click on the info button and it gives a brief history and details)...
In recent titles, when you check the info. about any RUF model, GT has never mentioned the name of Porsche (always refering to it as " another german car-maker)
But in GT PSP, they have actually used the word "Porsche"... so it seems they have acquired the license to use Porsche trademarks..
Far fetched, but maybe it's true..