Since the spring pad sits on top of the spring, it has a direct relationship to the ride height of the car. A thinner pad will most assuredly lower that end of the car.

I'd expect the change to be pretty close to the difference of the pads - 13 to 18 would be 3 or 4 mm change. I don't know the compression factor of the rubber pads, but as you surmise, there will be some.

My W126 has a self-leveling suspension (SLS) in the back. If I change the springs/pads, I need to adjust the control valve on the sway bar. Does the W201 have an SLS setup??

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FrankenBenz: '87 560SEC with an '02 cammed LS6 and custom 4L60E.
Bling: '87 560SEC, ported/polished, lowered
Lisa: '87 560SEL, lowered