If you switch to 17'' with 205/50 there will be no big difference for speedo, only a bit firmer ride, and better control through bends. I'd go for Renault original 17'' Steppe or Nervastella.
I've had my Meggy Grandtour '07 for 14 months now (bought with 17.000Km), 1.5 dCi 106HP, 70.000Km (43.000miles), no problems except for diesel leak on a common rail return pipe (strangely enough it was fixed by itself), numerous bulb changes (for that reason I now have HID 6000K), and the inside mir...
Mine also has the rain sensor, but the rain sensor is attached separately to the windscreen. There is this rail on mine too, but they couldn't fit it that way in the garage. They tried to glue the metal piece first, and then put the other part with the rail (as indicated in manual), but they failed....
Mine fell off out of blue. They just glued it back in the workshop using two-component glue. It is originally glued with some thick rubber-like glue. A good old piece of advice: do not fix things which are not broken. And good luck.
1.5dci 106HP, no problems whatsoever. Can hear noise of turbo at 1500rpm, but that's what should happen with the variable turbo around those rpm's. I had 35.000Km so far. I put ELF 5w50 and original Renault filter.