Thing is your basing an RS6 on track performance, that amount of power and weight will chew road tyres and even though they under steer I imagine they were still moving very quick, much quicker than a fast road drive.
As a road car they are bloody good, super comfortable for a car with this performance if not expensive to run!
Personally for a family performance wagon I would go S6/S4 Avant for understated power and cheaper residuals..