A guy on MeganeSport did out a normal Megane Coupe to look like a Megane RS. He fitted the 19" alloys too. I can't imagine he went through the effort of changing the hubs, but it might be worth looking through the project cars section of that forum to see if his thread is still there. (I can't remember his name sorry, but it should be easy enough to find).
Whats the existing wheel / tyre size from that should be easier to look at the difference it is going to make and you can work it out that way.
Looking at the project RS thread of ahmed24 (his username) appears they'll go straight on. He used a 21mm spacer as they sat so far in though against the wider arches on the RS project he'd built.