I can't comment on mufflers but I did install catted at design downpipes on my 2015 35is, but retained the secondary cats. Tone was deeper and I'd say 15% louder....not in an obnoxious way. Never threw CELs but we don't have emissions where I'm at.
I had a Remus axle back on a f30 335 with the n55 and it was a bit too raspy for me. Sorry, no experience with the eisenmann.
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