Well crap, I wish I could help more.....but I've only mocked up my AC Schnitzer exhaust system to check fitment. Its actually made by Remus per the stampings, but marketed or private labeled as ACS.
From what I can tell it looks pretty stout and probably won't be much louder than a stock exhaust with a tone increase with a hole or leak. Its all stainless steel, which adds tone by itself somehow. And its still a dual in exiting as one large square-round tip.
Oh......and its heavy as hell. It also utilizes the factory front most pre-muffler too.
From what I've seen over the years with factory replacement style aftermarkrt BMW exhausts......none of them are much different sounding really.......more of a tone increase is all. Unless thjey are made from mild steel which can burn out eventually and become louder.
You can always go upstream and add another muffler if its too noisy.
........Vinny