Yes thats the impression that its a resonator?Must admit, I’ve removed the bungs on day one of every zed I’ve owned and have never noticed any real change in road noise levels.
However, in a RHD car I am on the opposite side to the deeper roof motor well so it could indeed be acting as a resonator.
Remember the roof does still drain through there so is it a good idea to put anything in there that could absorb and retain water?