Ive never 'officially' checked it, but out on the freeway where I rarely do under 80mph I estimate it at around 25mpgs. Im basing that on when I go to the oregon coast which is approx 200 miles round trip combined and I have half a tank when I get back home. Thats with some steep grades too, where Im going into 4th and hammering down.
Keep in mind, the '03 models are tuned a bit differently, and have a smaller bore fuel injector. I havent installed my S1 yet which requires the big-bore injectors of the '04-up models (and will sling more fuel). As Im told, I can expect 23-24-ish give or take, depending on driving habits. Freeway running is easy mpg, and youre not really into the turbo too much, whereas around town youre boosting a lot more.