S5 drives like crap. There is this st. wheel vibration that many cars are experiencing and audi hasnt fixed - look on some of the forums and I believe it has been published. The materials and finish are nice thou.
My recommendation is to swap the tires first, see if you like the difference. If not, then go for the suspension.
'08 E92 335xi: the girlfriend
'04 Yamaha R6: the mistress
'99 Mustang Cobra: the wife