Well, a little more information.
Over the past weekend I had the opportunity to drive on a rough patch of road at night with a pre-LCI, Xenon E90 behind me. My lights were bouncing around, and no bouncing at all was detectable from behind. I followed an LCI car at another time - same thing. So, it's not "normal".
As I mentioned, I think a substantial contributing factor is the entire assembly bouncing around in the car. If anyone has a TSB# that's valid for sedans I'd like to hear it. I think next week I'm going to plan to put the front of the car up, remove the bumper and headlights, refinish my light lenses and look thoroughly for any improper assembly or opportunity to install additional supports under the lights when I put things back together.