I recently traded my 2002 S3 (Abt 250bhp) in for a 2009 RS6 Avant (stock 580bhp).
Completely different cars.
You can chuck the S3 about through corners, it's little ESP light flashing away. Do the same with the RS6 and you just get lots of understeer with all the weight, and you start heading for the hedge so you have to lift off!
The S3 is great little run around, the RS6 is a monster luxury GT car. No need to tune 580BHP right now, although Abt do a 700BHP package. Would still be slower than my Porsche though so no real point.
Biggest difference though is how thirsty the RS6 is. 80L fuel tank means £85 fill-ups and they only last 300 miles even hyper-mileing! The S3 would achieve 35mpg on a long journey whereas the RS6 will struggle to hit 22mpg.
Car parks are also a challenge in the RS6. It's so long that you'll come back to find someone has left you some nice expensive scratches. It's pretty hard to park in a spot where you don't stick out from the 'box' in some direction.
My father in-law has been in quite a few supercars and many times out in my Turbo, and he reckons the RS6 is the best car he's ever been in. Power, luxury, and practicality. The RS6 was always an dream of mine so it was certainly bought more with the heart than the head. Still, my dog is one of the fastest on the planet now I suppose