My IS250 SE-L has 19" rims and is on the American specification Eibach springs which give a staggered drop of 32 mm at the front and 24.4mm at the rear, so this may be better suited if you are towing. The European spring set is indeed 30mm all round and also gives a slightly softer ride than the American Spec (American spec has progressive loading whereas the European spec is described as linear).
The ride isn't particularly harsh, probably similar or slightly softer than my IS-F.