Something Worth Checking
My S3 has been running sluggish for a few days now. When driving home from work today the car completely stalled on the approach to a roundabout and left me with a rock hard brake pedal. I only managed to miss cars on the roundabout by using the hand brake and both feet on the brakes. After I'd cleaned my under pants I found that the vacuum hose which runs from the brake servo to the inlet manifold had split and popped off (on a 90 degree bend by brake master cylinder). Managed to do a get me home bodge with insulation tape. I've now made a more permanent repair while I wait for a new hose from Audi. The car feels a lot more responsive and has got its punch back.
Re: Something Worth Checking
I think there was a recall about 3 years ago, to get brake pipes replaced, but I'm not sure the problem was exactly as you describe. It would be worth making sure you had the recall work done though.
I'm not sure if this was a recall on the S3, but it was on the A3.