The car will definitely get louder over time. With my a3 I wanted some more noise so I went to powerflow to have the backbox removed and replaced with straight pipe to the bumper. Even that has got louder over the month or so I have had it on.
I'd give it 2-3k miles then see how you feel, then decide, induction kit or exhaust etc.
I've had a couple of S3s drive past me and they both sounded different, one quiet, one a bit louder so I assume one was just older, didn't sound loud enough for a custom exhaust.