S3 speakers are exactly the same as 1.8T speakers, i.e. shockingly bad. The only difference would be between a Bose & non-Bose system, in which case you can't just swap the speakers. You'd need to get the Bose amp, wiring & sub, and experience that classic muffled Bose sound.
The standard Concert setup in an A3T is one of the worst I have come across in a modern car. My old MkIII GTi had a much better setup, and even the Astra I had (as a Co. car) was significantly better.
You can retro fit Kenwood speakers in any A3 for about £130 (fronts & rears) and get a significant improvement.
Also Dynamatting the doors and rear quarters will improve the tightness of the sound at the same time.