Yesterday I finally got around to taking the Commodore to the mechanics to work out what's wrong with the power steering. It had a belt squeal, and the power steering assist got weak at low RPMs, so I figured it was a slipping belt and replaced it as part of the 200,000KM service. Regrettably, while that shut the belt squeal up, it did very little to fix the power assist.
I figured it was the P/S pump, but I didn't want to just chuck more parts at it and have it not fix the problem, so I took it to a professional to diagnose it, who came to the same conclusion. We ended up chucking a used pump on, because they're about a third of the price and quite frankly I've spent enough on car repairs at the moment (it looks like the Sprinter has done the clutch fan, which will need fixing in rather short order).
Not only is it better at lower RPMs, it's better all around - I didn't realize how bad the power steering was until we got it fixed. Even cornering is a lot nicer! I'm actually looking forward to the next drive out to Mum and Dad's to see if it's better in the twisties.