I park my '97 2.8 next to a '98 2.8 in my parking garage at work, so I have a "stocker" to compare it to visually. One thing I've noticed is that my Z3 rides about 1/2" lower than the stocker. It piqued my curiosity to the point where I measured ground to rocker panel by the wheel wells, and I was indeed sitting about 1/2" lower. I measured the wheels to see if that might be the diff, but my wheel/tire combo is actually almost 1/4" taller. The only thing I've done to my suspension so far is to add swaybars, which I don't believe affect ride height.
Do cars settle after a while? Is it manufacturing tolerences? Is it the crushing weight of all the mods I've done?