We just went backwards and forward recently on the lower deck in the Aero Coupe which should be the same 4.8L exposed sump as your S5. No concerns re the grounding,
Lower deck is fine (narrow) but fine. But it was the upper deck he asked about - which in my experience is most definitely not fine for low hanging engine vegetables.
It's the hump at the end of the ramp which is very severe.
