What they will have done is to replace the rubber coupling in the steering motor as per the TSB I mentioned before. They shouldn't have needed to touch the rack so steering geometry shouldn't have altered. Assuming that your light steering problem only arrived after the repair, the most likely cause is the wrong programming of the column motor controller.
When the column is re-assembled it has to be set up on the dealers computer system, and there are different tune downloads depending on vehicle spec as the same basic column is used on allCeeds. I should get your dealer to check which tune they downloaded.
The steering assistance levels are set up in software at the factory or via the dealers programmer. They are correct that they can't just invent a new tune. However, different models of Ceed do have different settings, and European settings are also different from elsewhere. I believe they would have to electronically reset the column when they did the fix, so could potentially have chosen the wrong settings then.
You're not having much luck with that car are you! I agree about the Michelin tyres. I had an MG ZT previously and couldn't understand why everyone raved about the Michelin Pilot Sports that had. I took advice from the tyre test people at MG Rover and changed to Continental Sport Contact 2. What a difference! Better wet grip; better ride quality; massive reduction in tyre noise. I can't comment on snow performance unfortunately.
No guesses for what the Pro-Ceed will get next time round.