That is why I love the test pipes or x-pipe/HFC option. Both essentially give you removal of primary and a 200 cc cat left. Although HFC uses metallic 200cc which significantly has more flow than the OEM ceramic 200cc. Point being the decibals may not apply exactly to a HFC setup/x-pipe which I have but I am guessing very close.
Reason I love this is because it truly is not "loud" It sounds stock in every single way in terms of drone, low rpm drone, loudness at low speeds etc. It truly only gets moderatly louder but more importantly the quality of the sound gets higher pitched, raw and raspy-sounds like a race car without being that much louder.
I think the m3 exhaust system, just like their intake design with 3 routes and a huge cone for the coolest air system ever, is reallllly hard to ever improve on. The system is so darn straight back, piping diameter is perfect, xpipe is great and the rear section despite crossing over in the middle provides extremely good flow with effective design to keep the drone out and the higher pitched sounds in.
So the only flaw which is not bmw's is the primary cats like any car robs power. So changing those, or getting an xpipe/HFC and leaving the OEM rear just makes a lot of sense to me if you want a race car sound that is not intrusive or completely overbearing if you deal with high end clients, resorts, restaurants etc. Anyone that does knows that you stick out like a really trashy person if you have an obnoxious exhaust blaring out of your car when your pulling up to meet two potential business clients at a nice restaurant. This is stealth and why I love it