Forgive me if this sounds like a n00b question but given the weather you've had there the last couple of years meeting mid-August what is the weather like there in June and July and would it be possible to make it happen then for a chance of better weather or is the 'Ring unavailable then?
This year basically every day in the summer was like shit here. And the Eifel is quite special itself. According to my mother she had a beautyful sunny weekend with 25-29?C friday till sunday, and that just about 200km south (air-line)...
The only thing about the A6 3.0 TDI is, that it takes it a long time to get to that speed.
I had one of these in front of me on my way back. Traffic was tight and he behaved like a typical Audi driver: Flashing, blinking, tailgating. Then the Autobahn went from 2 to 3 lanes and I couldn't resist showing him superior power.
Sorry for the bragging but he really was no match for the mighty 535d. Not a chance in hell.