as a repairer off cars i do remember a lady some years ago had a 3 series compact,unladen it was perfect,as soon as she had rear seat passengers it "dragged its ass" Unladen on a level floor with no broken springs it looked perfect.she wanted somthing sorting out so we changed the rear springs for OE spec springs.measuring before and after there was a negligable increase in height,5mm max.she had the car back and a week later reported it was perfect,no sagging.we deduced that in the course of its life it had incorrect rate springs fitted.an easy check is to inspect for paint mark,eg brown brown blue and check they are the correct spec.a bmw dealer would also be able to supply the correct colours for uprated springs which could be purchased aftermarket