These cars do not easily step out on the rear ... If anything it should understeer in the wet rather than oversteer.
So I'd definitely say something is wrong ... Was it a garage who fitted your springs or yourself/friends?
Had them fitted by a local garage i have been going to for years, but he did have issues fitting them, took them 5 hours and i had to have two new top mounts on the rear!
Hopefully the garage tomorrow will sort it out or at least find the problem.
If not Stealers might have to be contacted...:(
Do they like modded cars?