Porsche asks for the rear seats to be removed, this is no recommendation but part of the official approval for the GT3 seats. It has something to do with rear passenger safety.
So if somebody gets hurt in the rear (like the teeth "kissing" the hard shell of the front seat), you might have problems with insurance, etc. Not worth it. And yes, even if only the driver seat is a GT3 shell, the rear seats must be removed.