Big turbo also gets my vote.
People have asked me and I have considered myself why I dont just get a B5 RS4 or something thats already quick.
The thing is I can't honestly think of anything within budget, £12k, that could what I want a car to do better than spending £5k on a big turbo setup.
You know your s3, what you've replaced/upgraded, what's likely to go wrong and the best part is when stuff goes wrong it's not that expensive to fix.
I.e. there are C5 rs6's around for £13k, if the gearbox blows it's a £5K job on the s3 it's between £500-£1K.
Any serious work on a B5 RS4 is an engine out job. $$$$$$$
Evo's loud and noisy all the time, but big bang for buck.
My s3 is quiet on a motorway journey or when pootling around town but sounds awesome when giving it the beans, Its comfortable, you can use it all year round, there's loads of specialists around to do any work for a good price (although theres only one that counts), It's not massive so it still handles well, anything that goes wrong is bound to be used on at least 2 other cars that VAG make so parts are easy and usually fairly cheap.