Ahh I though all GT3's had the plates for the cage and the holes in the carpet / trim.
Wiechers: Good call! I'd forgotten about them. It says in their advert that no modification is required. I wonder if they just mount through the normal skin. I don't like the idea of a cage without reinforced plates.
It looks like I'm going ahead with this car 8) . It also has normal GT3 seats (for now), so the GMG cage is sounding more attractive, as it's positioned differently for extra clearance behind the seat.
The other alternative is to have a proper bolt in cage fabricated by the guy I use for my competition cars at Nickson Motorsport. It's just a bit of a pain to get the car there and back to be done.