It seems to me that  GTS is a model that Porsche releases after Facelift and at the close End of the Model Cycle, to push the sales to the maximum possible. Its a special offer (for what you get) in a great package. As Seen on the Cayenne 1 and on the current 997.

So following that strategy, dont expect to see GTS in the next 3-4 years. And than the GTS question will arrise again, why to buy a new Car, when you know that in 2 years the successor will come?!