I think it is generally accepted that a stock RS6 in good condition will run a 13.1 sec 1/4 mile and 0-60mph in 4.8 secs.
There are some exceptional stock ones that will break into the 12's.
Car and Driver did an extensive series of comapros with hot sedans like E55 back in 2004 and could not get the RS6 into the 12's.
Inability of RS6 to brake torque and thus getting off the line and the relatively 'slow' first 60ft acceleration time was the killer. The RS6 loses half a second inthe 1/4 mile time at the start. just my two cents worth. my best time standard was 13.01 secs for the 1/4 mile.