Update - did the mod (the panel was just a pry-out, nothing fancy involved, though I recommend trying to pry it out from the deeper end, rather than the one closer to the trunk opening, as the clips there are straight instead of angled). That being said - the sound is definitely quite different. I like it better this way. I can understand why BMW wouldn't do this by default, though - this is NOT the typical sound one expects from a BMW - this is more of an "enthusiast" WRX / Evo / whatever sound. I don't see an average mid-40s family man buying a 335xi and liking this - he'd probably call it "loud", or whatever.
That being said, I paid attention to cars driving this morning: it was a few degrees below zero, so you could easily see smoke - and all the 335s I saw had exhaust coming only from the passenger side. I found it amusing, and realised I never paid attention
Thanks OP!