I would personally go with an aftermarket Magnaflow. I got a brand new one put in for $250 about 18 months ago. A good shop can have it done in 30mins so its not a big deal. I've 'heard' though this year they changed some cat price laws in Cali so drive to a few places and get a price quote. I noticed no 'noticable' difference in exhaust note from the magnaflow cat either...but then again I had a few holes patched up too...so take it for what its worth.

Burning oil will ruin the life of aftermarket cats quicker than an MB cat though. As long as your oil consumption is ok I wouldn't worry about it. Save your cash now.

I just passed the CA smog a few months ago fine too.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
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2003 Gray Mercedes ML350 - 122k