I have some stats for you from the now-defunct but well-respected Audi driver magazine. The point here is that they had a repeatable road-test routine that always included the test track at MIRA and the same touring route and the same road-tester, the late Paul Harris, using the same techniques.
Their road-test A6 BiTdi was down on power and gave poor acceleration figures so I am including those from the A7 BiTDi tested in October 2012 and other comparable models tested by the same team.
A6 BiTDi 0-60 6.3, 0-80 9.6, 30-50 in 3rd 2.0, 50-70 in 3rd 2.9, overall fuel consumption 37.5
A7 BiTDi 0-60 4.8, 0-80 8.2, 30-50 in 3rd 1.9, 50-70 in 3rd 2.7, overall fuel consumption 34.6
S7 420PS 0-60 5.0, 0-80 7.7, 30-50 in 3rd 1.8, 50-70 in 3rd 2.6, overall fuel consumption 21.0
RS7 560 0-60 3.9, 0-80 5.9, 30-50 in 3rd 2.0, 50-70 in 3rd 2.1, overall fuel consumption 23.1
RS6 560 0-60 4.2, 0-80 6.1, 30-50 in 3rd 1.9, 50-70 in 3rd 2.5, overall fuel consumption 23.3
From the above you can see that the S6 will give roughly the same performance, give or take, roughly 15 mpg less as confirmed by DonkeyIce, and the most fabulous soundtrack to equal any AMG Mercedes.
Yer pays yer money......