I just noticed, on your first picture it appears the black plastic piece that fits between the headlight and beak is missing. It fills the space between the beak and the headlights. The panel was originally fitted by BMW and should have been re-fitted with the tank. This item is shown on page 8, photo 23 on the Touratech installation instructions. These instructions are listed for download on their website if you don't have yours. I think that panel prevents the problem you have heard.
First picture post 47 shows the part I am talking about.
Hope this solves the mystery