Hi, I've got a Q8 Etron 50. Keep in mind that the lowest spec on a Q8 is more like a mid to high range spec on the Q6. There are no options for RWD only and all cars get the dynamic adjustable air suspension.
I've had 250 out of mine quite easily, and mine only has the 95kwh battery. Id completely expect the 115kwh battery of the 55 to be able to hit 300 miles real world range. Whilst the Q8 etron is quite an inefficient car - the 'cure' of a whopping great battery has advantages in other ways. I just completed a trip down to the middle of France, and I averaged 2.6kwh/m whilst doing 130kmh on the motorways. What hurts this cars economy most is stop start driving. It's a big heavy luxury cruiser and it's been made to excel at that.
Also, regarding charging speed. I've yet to find a charger that actually delivers the 175kw that the Q8 is capable of charging at. So charge speed is already very good on the Q8, so not really an issue.
The Q6 has only just been launched, so real world experience with the car is low and I doubt there will be many good deals on the car. Based on owners groups and current etrons, I'd avoid the panoramic roof (unless it's the non-opening sort).