audi s5 suspension?

I have an Audi S6, and the suspension is definitely more stiff than the A6, but it makes for a more fun drive in my opinion. In my opinion you could take your S5 out on the track the way it is (stock). However, if you're looking to improve it further, the first thing I'd recommend is a quality set of coilovers, the lower the car is the better its going to handle on the track. I'd also upgrade the rear & front sway bars, as these Audi's, or at least my S6, is very heavy.

I wasn't sure if you meant performance upgrades as well, but for that I would have to know what year your S5 is to determine which engine you have..

Just my 2cents.

As Dan mentioned, coilovers are the way to go but keep in mind, on the street you will be dealing with pot holes and less than perfect pavement. If you slam the car on a stiff suspension, you best make out a budget for bent and cracked rim replacement.

I completely agree with Dan's answer.