I thought Whoopsie had settled this question for us. GT4 and Spyder get a 6 cylinder motor, but not a detuned version of the GT3 motor; rather, and updated version of the 3.8 in the previous GT4. 

The dealer will of course say the GT4 and Spyder may get a version of the 4 cylinder motor, so they can keep their 718 stock moving...