So I removed my secondary cats a few weeks ago (I already had the primaries removed) and Oh My God...the exhaust became loud as hell and it had a mad rasp, you cannot imagine how bad it sounded. Between 1800 and 2000RPM, in neutral or under load, it sounded like a lot of metal parts were grinding very loudly against each other. Basically like running open headers. Drone was not existent though thanks to the BMW Performance Exhaust.
First I replaced my OEM catless downpipes (cats replaced with pipe) with Macht Schnell 3" coated downpipes. I couldn't feel any improvement in performance or in sound, although maybe the rasp moved a couple hundred RPMs lower in the rev range. The downpipes fitted absolutely perfectly and the ceramic coating looks extremely good. The welds are also perfectly done, I am very happy with them. For $610 coated (I got a deal but it's not hard to get such a deal from MS), I think these downpipes have the highest quality/price ratio. I highly recommend them.
But what about the rasp ? I immediately ordered a pair of Vibrant 1792 resonators, and I welded them where the secondary cats used to be. I honestly didn't expect a big improvement because they are quite short (12") and straight-through inside and they couldn't possibly improve such a bad rasp and such a loud sound. Wrong ! The rasp is 100% gone and the exhaust is much quieter, identical or less loud than when I had all the cats installed ! It's absolutely incredible how a pair of $45 straight-through resonators have such an enormous effect on the exhaust note and loudness.
Conclusion: I am extremely happy with my setup. The BMW Performance Exhaust + Macht Schnell catless downpipes + Vibrant 1792 resonators produces deep, drone-free, rasp-free and not too loud sound while maintaining the performance of a fully catless setup.
Below you can find some pictures and a video of the sound.