Even the standard S3 seats are pretty hefty.
Biggest interior weight reduction would be from removing the rear backrests and the bench. They probably weigh close to a small adult. Replacing the front seats with a set of lightweight sport seats would probably get you a good amount of weight saving too. Other than that, you could start removing aircon, replace windows with plexy glass (or whatever they use in race cars).
The plastic trim is pretty light, but you could go to town on that too. Ultimately though, the S3 is heavy mostly due to the haldex system in my opinion, so for all out lightweight car it's probably the wrong base to start with.