You are correct, like the TD's, the rear springs are softer, since the SLS struts have to take some of the load. In other words, if the rear springs were the same as non SLS, how could the rear adjust down to a normal height when the trunk, back seat, fuel tank is empty?
Neat car, though. Someone ordered that to tow something. Couldn't have been cheap to get SLS as an option. Are you saying it has front SLS? I thought that was only available on the 116 in 79.