I'm contemplating a GT3 vs. C2S. Back seats are somewhat of an issue, because I have 4 and 1 year old children. I went to the dealer with my Britax booster to test it. It does not fit squarely on the seat bottom, but is better with the bottom cushion removed. Even then, the passenger seat would have to be all the way forward to fit a pair of legs back there. Forget the seat behind the driver, unless your passenger is totally sideways. I understand Porsche has custom child seats, but I don't think they'd solve all the problems. Bottom line, the rear seats aren't very practical for humans.

BTW, it is legal to use a child seat in the front passenger seat, but it is not as safe for obvious reasons. The airbag has to be turned off. Porsche can install a switch to do this.