I just made an impulse buy on eBay for or a set of black XKR Nevis Wheels for my black 2010 XFR I had looked at them, saved them on my Watchlist and then received a -10% off offer. I have an extra set of brand new Extreme Contact tires to mount on them and I wanted the option to change the look up a bit but still be able to go back to the stock look if I didn't like the all black look. The listing said also fit the XFR and it wasn't until I hit the buy it now and paid that I realized that the fronts are the same but the rears have a different offset 46mm vs 66mm for the XFR. Wouldn't this be the equivalent of having a 20mm spacer on the rear setting the rear wheels more flush with the rear fenders without having to use a spacer like many do. If this is the case then I am fine as I was considering getting spacers anyway.
. I believe the bigger issue would be if I had an XKR and tried using the XFR rears as they would sit way to far in and would take a spacer just to get back to stock depth.

 

You are correct, A et66 wheel with a 20mm spacer has the same FINAL ET as an et46 wheel.

Wheels with the same width but different offset will change the fitment relative to the fenders (and the inside running gear/brakes) In this case since the wheel design is the same and assuming the width is, the spoke angle/concavity might be different. So one thing to mind here will be brake & suspension clearance on the XF vs the XK. You are moving in the correct direction for more clearance but its hard to know without a test fit or drawings of each wheel.