Weird that your '91 has that setup? What options do you have? Sunroof? My '91 300E has a sunroof and came stock with 3 bump up front, 2 bump rear. When I changed my springs, I lowered both ends a bump (2 front, 1 rear) and am happy with the results.
Appparently it depends on the weight of the car when it comes off the assembly line with options factored in.
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi