A 190/55R17 on the rear is your best choice for a nice rounded profile that will allow the bike to flow into corners...The taller profile given by 55 also means a rounder shape. Somewhere I read a comparison between the 3 possible rear size choices for an RC51 and this was deemed to give the best handling & feel.
I also do a fair amount of motorway miles and find a BT016 on the rear and a BT003 Race on the front to be a good combination that will wear roughly within the same mileage. These will also be great tyres for country scratching.
It's good to change both tyres at the same time, but be really careful for the first 50 miles or so as the release compound on the tyres can be very slippery to begin with.