High flow secondary cats

A couple of months ago, I bought a slightly used ets exhaust system for my e92 335i. Its a dual 2.5 inch system with an x pipe, and separate mufflers/exits. I was running the factory e92 335i catback before with stock secondary cats in place + catless downpipes. Now that I am fully catless, the more freeflowing exhaust has def provided a nice midrange gain....and it actually idles around the same sound level as the stock exhaust did, only maybe a tad bit louder.

Thing is, at first I was alright with the fuel smell, and I kind of still don't mind. The interior pieces of the car do tend to make noise/rattle at a certain rpm due to drone, mainly the center console. I rode in a friends rb turbo 335i yesterday who had a custom dual 2.5 inch exhaust to an x pipe, to bastuck muffler, with some 300 or 400 cell cats welded in the midpipes. Not a peep as far as noise is concerned from the interior pieces of the car. No drone. Before, without the cats, he said his car droned horribly.

Question is, I found some 100 and 200 cell race cats from mbs. I want to restrict flow as little as possible, but am unsure of which cats I should order that would help eliminate some of the drone/interior rattling. Any ideas or experiences?! Thoughts? Thanks!

http://www.mandrelbendingsolutions.c...High%2C/Detail