On my TTRS they leave the entire driveshaft tunnel and exhaust uncovered. There are also cooling fins on the rear diff / haldex so I'm fairly certain I shouldn't cover those up. But I like the idea of enhancing the LCAs.
I also purchased another right side rear cover for the left side as they are almost interchangeable. I'm doing that mostly for fuel tank protection as there are some nice "chips" in the plastic tank. I don't think it was left open for any kind of cooling but it's really hard to know why it wasn't covered. I suppose the left side fuel tank itself acts as a cover as it's flat but there is a gap between the side cover and tank. The factory exhaust also blends into the rear diffuser/bumper for decent flow.
The whole exhaust tunnel is left open (in your case) for a reason.....
There was a UK forum member earlier on in this thread that had a car like mine (2wd & one exhaust pipe).....& he made a custom full aero tray with some air ducts to completely cover the exhaust tunnel from front to back..
He removed it awhile later as he noticed that on long journeys the interior trim in the cabin on the tunnel was getting "warm"....
You have two exhaust pipes & one prop shaft.....leave it "as is".
However, I notice that you do not have the MK8 Golf rear lower arm aero covers...these will improve the aero & quieten down the noise in this area.
I also note that your car has the front arm air guides to the brakes & you have a full aero tray for the engine bay fitted...!!!
A few pages back there are part numbers for the "Eco" MK8 HUGE front engine bay tray which also covers the front subframe & works with the Audi Alu subframe which you should have.......word of warning...you might have to cut some NACA style ducts into it as its made for the "Eco" engines..so not much call for heat/cooling & therefore has no ducts....