open throttle on a bend and it seems to want to drift the corner rather than pull you round it.
That shouldn't happen, if as you say, you had the throttle down sending power to all four wheels (50% F/R with the haldex system). The S3 is a nose heavy car as standard and the safe handling should see it neutralise into understeer if cornering too quickly, certainly not oversteer.
Couple of scenarios where you might expect to see what you explained:
1. Lift off oversteer. You take your foot off the throttle at the entrance/part way around the corner, the rear will come around. This is basic physics not helped by the Haldex which doesn't operate unless you have your foot on the throttle.
2. Your rear tyres are worn. I had this issue, replaced tyres, all sorted now. Also worth checking suspension and bushes.
3. Slippery road surface causing oversteer. But one would expect the front to wash away into understeer way before having a sniff of oversteer.
Your car simply shouldn't do that. I would check the main things first, tyres for wear and pressure, bushes etc. The Haldex unit will push 50% of the power to the rear axle when needed, no more than this. Next time it's wet find a large open space and drive around in a circle, slowly gathering speed. If all is well then the front should start pushing wide way before the rear.
If you find the car is oversteering before understeering, you have an issue. And I want your car.