Here is a set of K03 turbos fresh from a rebuild which included blasting, new hardware and new balanced cores from The Turbo Unit.
These units looked amazing once they came back considering the state of them when they came off the car. In this case, the decision was taken to stay with the smaller K03 turbos rather than do the more traditional K04 upgrade.
The K03 is a small turbo and they spool up very quickly. Coupled with the large V6 in the B5 S4 it means the power delivery is very linear and useable, which is desirable in a road car. This car also received new downpipes with no pre-cats, 100 cell sport cats and a full stainless exhaust to help it flow. Once is was remapped to stage 2 it built power very quickly, and made well over 300hp on the dyno. In fact, it made nearly 300 on the stock ECU map with just bolt-on upgrades. Yes, there is a sacrifice on the top end versus the K04 upgrade and a K04 car will eventually run away form a K03 car, but I think that this car could get a jump on a K04 car off the mark.