OK, you "can" get 24.5mpg with an RS6 on the highway. Absolutely no boost and nothing over 60mph (which equals no fun).
I averaged 24.5mpg from Redding, CA to Portland, OR along I-5 after getting a ticket just outside of Susanville, CA where he clocked me at 81 (I was slowing down from 100+), so I was butthurt and decided to go super slow.
The other question to ask is - why bother worrying about mpg?!?!? Its miles of smiles!