The S6 stock is capable of low 12's in the 112 mile per trap speed range. So, just about another 7 miles per hour added showing that the tune by itself is making quite a difference.
Tuned S6 and S7 vehicles definitely are a value alternative to the RS6/RS7. Additionally, unlike the RS models the S models offer a dual clutch transmission. They also offer the same displacement from the 4.0 TFSI motor although tuning and turbo changes explain the horsepower difference.
Car and Driver recently showed the stock RS7 is capable of 11.6@123. So, a tuned S6 is able to match the elapsed time thanks to the dual clutch transmission but not the trap speed of the RS7. Regardless, additional bolt on modifications (exhaust, downpipes) should bridge the trap speed gap further meaning a tuned S6/S7 does not give up much of anything to a stock RS6/RS7 making for a tuning value.