Yes, CZ limit is 20kg total, not per bag. I've never been charged for excess luggage but the fee of RMB46/kg sounds about right

Interestingly, up until this year I never encounteres a major problem on Chinese domestic flights with 23-35kg. Sometimes they would mention I was overweight but usually I could come up with a decent excuse. Most of the time they never mentioned it. 2008 is different, I've been pulled up even at 21.5kg and had to repack some things into hand luggage