I put on 215/55-17 snow tires, no problem, in fact I think they "fit" better than 225/50-17 OEMs.
Remember variable valve timing can kick our engine into and out of Atkinson (late intake valve closing) at will, although its true efficiency does elevate when Atkinson vs. the usual Otto cycle. Power is down when in Atkinson, but efficiency is higher.
The ICE in the C-Max does NOT switch from Otto to Atkinson. It is always in Atkinson mode.