312K is a pretty good run for a set of springs.
When folks state that they 'never wear out', I don't think they consider that a car might last this long.
If you do get new ones, it would be very interesting to get back comparative data on the new vs. old uncompressed spring length.
Please let us know.
I would expect that the rubber spring mounts would also collapse over time to some extent, which might account for some portion of your drop.
Of course, once you go through the trouble of pulling the springs, you may as well replace the CA bushings, and cast a critical eye on those balljoints. At that mileage, you are probably due for a third round on these components.
Best of luck.
1986 300E 5-Speed 240k mi.