I have 17" wheels -- I think with the 163 PS cars the wheels are also narrower. No doubt this is for reduction of rolling resistance to get under the magical 100g/km in the lab.
Anyway, I'm more than happy with the tyre noise level. I was something that I expected to be better than in German cars and I'm pleased that, from my experience, it is. I travel to Germany a lot, because my wife comes from there. Their roads are typically in a better state of repair and they use a finer surface dressing or so I believe. I have driven Audi's & BMW's in the UK and while the engine is quiet, the tyres are roaring away. I would say that Mercedes generally pay more attention to coping better with rougher road surfaces.
Of course someone with with a recorder/analyzer could prove that the above is a load of subjective tosh