All, 

f pace wheels have a 45et, my xf has 40et...what happens if i change to 45et...

still trying to get a softer ride and these f pace come with 255/60/18

paul

ET is the distance between the mounting surface and the centre line of the wheel. So if they are the same width and a smaller offset then the inside edge of the wheel will move outwards. If the width changes as well then its more complicated. Assuming you have 255 width wheels on there now then the f pace wheels will fit (assuming they are the same fitment). 60 is a very tall profile and on a larger wheel. so your biggest problem will be that your speedo wont be accurate anymore as I'm sure they will have a bigger rolling radius that your standard ones. I.E. you will be travelling faster than your speedo thinks you are. Not only could this incur speeding fines but its likely to confuse your gearbox and ruin your fuel consumption.

In short, unless you want to spend a lot of time and money making everything work with the different wheels.... don't bother.