Can't comment on the E(x but with previous cars, the ride difference going from 18" to 19" is massive. 16" to 17" is barely noticeable, 17" to 18" is noticeable but still relatively comfortable. Go to 19" or 20" and you may as well replace the suspension with blocks of wood.
19" look the nuts but don't kid yourself that it's going to be a comfortable ride, especially if you're on 18s now.
Change your RFT for normal tyres on the MV3s though, and your poor, jarred back will thank you every time you get in the car