I reckon the same - what did BMW put there?
On my car there are a few M's on things like the steering wheel and door kick plates and so on but the badge on the back (if it was still there) would simply say "330i", because it's an M sport model of a regular BMW, not an actual M car.
BMW themselves are no blurring the lines slightly with the M135i, but that doesn't change my opinion.
If BMW put an M on it at the factory, it's up to you if you want to keep it or not. If they didn't put it on at the factory, why are you trying to make it look like something it isn't?
Take what i say with a pinch of salt though, i'd rather have an M3 that looked like a 320d than a 320d that was trying to look like an M3...