I think to get a V8ish sound you need those oval-shaped tips, or you can just go for the M3, that has a real V8
Jokes apart, I know what you mean, I once drove a CLK base model that sounded like a V8 even though it had a V6 under the hood. I think it'd be harder to get that note from a bimmer. It might be because straight sixes sound different than V6s. I might be talking non sense here but for what I've noticed bimmers always have a sportier sound (higher pitch) and Mercs have sort of a muscle sound to it.