The rear springs & shocks would fit between XI and non-XI. The front struts may have a different mounting location for the sway bar endlinks, otherwise they would be swappable too. You can buy adjustable length endlinks though, so maybe that would solve it if there even is a difference. As far as differences in the actual springs & dampers, I would imagine they would be tuned for the added weight (220lbs) and thicker roll bars of the XI. Don't know how much of a difference that would make though.
2003 325xi Black³