The S3 is no where near as bad as the ST for fuel. Mixed driving, expect 24 - 30, thats sensible driving, with a few spirited corners and acceleration thrown in. Enjoying the car so to speak. 300+ miles out of 55 litres.
But be warned. The S3 as standard, is not a hardcore handling car. It will understeer like a pig as standard (you either have to learn to drive around its limitations or upgrade components), and arguably the ST is better performing in this respect.
But it is an excellent cabin to be in, refined, and won't make your ears bleed or give you a headache after a 3 hour journey. It is a quick straight line car, and the Haldex makes it worthwhile in the winter and slippery roads.
A post 58 reg facelift car. if you can afford it, is the one to go for, this has the Gen 4 Haldex fitted.