Ran into this 992 GT3 RS Roof Fins on Suncoast Porsche and very interested, also confirmed it could fit on my 991 GT3RS and other GT models as well.
I did some research and there are carbon ones available from other brand but those are very pricy. There is one vendor on ebay that brought to my attention.... I really don't want to see people from Porsche community to get ripoff by this seller/vendor. Suncoast Porsche is $815 for OEM Roof Fins where this seller/vendor charge $1499.
If anyone also interested on this RS Roof Fins, Suncoast Porsche is the way to go.

 

Why would you get or need these? Those fins were installed as a consequence of the RS's huge hood nostrils to redirect air flow away from the rear engine intake. That's it. Not needed on any other car.

 

Yes, you are correct. This is just for look only, i decided not to get it as you mentioned there is no benefit to the car.

 

Aerodynamic appendages on the roof are to stall the lift of a car when it goes sideways. When a car is sideways all of the spoilers & vortex generations lose their effectiveness. Adding to the fact that you loose your downforce, the shape of the car when it turns 90° produces lift.

This nascar video talks about their flaps but the ribs produce a similar effect of stalling the lift.