Assuming you were able to negotiate a 5% discount or better, and are paying mid-60's for your car, I'd say you have a very thoughtfully spec'd Macan S.....Should I step up to the GTS and pay the price difference? .....
Given the duration you plan to keep the car, and the lack of performance options on your current S build, I would say don't get a GTS, and stick with your original S build.
All of us have seconded-guessed ourselves after placing a factory order. But usually, all is forgotten once you receive the car. In any configuration, it is a fantastic car.
Have a look at these threads for some insight on how good a bare bones Macan S can be: