i have 275/40/18 ... diameter 26.7 ... your proposed 285/35/19 is 26.9 ... should fit the wheel well with no rubbing ... and the 30mm extra width is easily distributed on the width of the xF rear wheels so that should not be an issue other than making the mounting process a tad harder.

edit... make sure to reply back with how well they fit a few pictures if possible.