Don't compare the Boxster to the BMW's. The E36 and E46 have really weak rear shock mounts. I know I have an example of all three cars.

I would echo checking for aftermarket springs, my 98 when stock was pretty plush for a sports car. Also as noted tires are springs and make a big difference in ride quality.

Is the ID tag still in the inside of the trunk lid? The parts list I had off the internet does not say exact years but the id does look like -99 (before 99) could have a M030 code. If this number is on the tag you could have a heavier suspension installed and not know it. Heaver spring, struts, and swaybars. Just another thought.

Check the DOT date codes on all tires if they are older than 4 years that is contributing to your problem. Also look for small cracks in the sidewalls especially close to the wheel rims. What brand & size tires do you have? Springs is the other thing to check.