The difference between the GT3 shifter and the regular shifter in the Carrera are the bushings. In the GT3 shifter, the bushings are made out of metal versus plastic. This gives the driver a more direct feel when shifting. We measured the throw-action of both shifters and the throws are nearly identical. For a shorter throw, we recommend getting the Porsche SSK (Short Shifter Kit). 

On the left is the GT3 Shifter, right is the stock Carrera Shifter

After experiencing both shifters, we can say that the GT3 shifter gives a more direct feel compare to the Carrera shifter. Every shift is improved, making it a more enjoyable driving experience.