Other than oem the only other converter I
heard of with BMW mentioned in the same sentence are DEC catalytic converters. They are rumored to work well. I think they are around 350.00 each, I priced them a while ago for my 840. On a side note I took my car in for a smog test in New Jersey today. They got rid of all safety inspections just smog now. Great! Junkers galore coming to a theater near you! I was nervous I was going to fail with a nikasil block and 185k on the motor. My co was close to the limit 2 years ago. I passed! They don't even give you the results anymore! Would of liked to have seen them. Anyway I'm sure you know that the oem ones are expensive. Good luck!
Great to see an original member..Thank You