I had exactly the same problem with my 330d for the first few weeks - i.e. it always felt like it was running too slow for the amount of rain.
I then had my car detailed and winter prepped, including getting all the glass cleaned and coated with Gtechniq. Since then, the rain sensors have behaved differently and I now keep them on either minimum setting or one up from minimum and all is fine.
So I guess the idea of keeping your screen clean around the sensor works.