Well, not really, no.
The point I'm trying to make is that if there is a risk of any roll cage coming into contact with your head in an accident, then you need to be wearing a helmet. I do not know if the GT3 cages are designed so that they sit far enough back that it's not an issue or not, and I'm not saying that the 350Z cages definitely sit close enough. I simply suspect they do, given the enclosed nature of the cabin.
Convertibles with roll hoops/bars almost always have them situated so far back that contact is impossible, or at least if it happens you've got bigger issues i.e. complete deformation of the car. They need to be that far back to make a pivot point with the windscreen so that if the car is flipped, any head inside will not be able to touch them ground, hence the incredible strength in windscreen frames in droptops. That's 101 in OEM car design. If people choose to fit aftermarket hoops/bars to their cars that could bring the head into contact with the bar in a roll, then my point is equally valid for that.
Really, it's all about common sense. If you've added a shed load of metal tubing inside the car, you'd better be damn sure that it's not going to come into contact with your head when you roll the car, else all you're actually doing is making the car more dangerous. Certainly if helmets are not going to be worn then harnesses absolutely must be: Caged cars with no helmets and just seatbelts is an incredibly dangerous combination. That much is plainly obvious.