Was also going to raise spacers as a suggestion, but conscious of the worms in that can !
It’s strange in a way, as I ran spacers for years on my 5 series, as it seemed did almost every other member of the forum at the time. You could guarantee posts on the best sizes and set-ups on an almost daily basis. Notwithstanding the potential stresses on suspension linkages and geometry it seemed to be an accepted given, and must admit I personally never had any issues.
Yet the IS has a huge gaping void in its arches (especially the 17s on my Exec) so I’m surprised the spacers query doesn’t appear more often. I even have a spare set of Eibach 20mms in the garage but have never put them on the IS. Perhaps the additional battery weight over the rear accentuates the stresses in a hybrid setup.