Not sure if anybody posted or used this product before on this forum. I was introduced to this product called Guaranteed to pass (G2P) by CRC. It has helped some of my friends' cars to get rid of the check engine light for cat/emission to pass inspection tests. This might help some of your cat problems that we experience on our 318ti. Could be a fix for older cats that just needed to be cleaned up from years of buildups. Of course it can't fix a known mechanical cat problem. For the price it's worth a try first than spending $$$ on a new cat or 02 sensors. It can be purchased at your local auto parts stores. What do others think about this product or experiences with it?
http://www.amazon.com/CRC-05063-Guar.../dp/B000CIPUR8 
Might help some of your CAT problems 
Linear Mode
