Another thing about Columbus: the alloy was stronger. Columbus Cr-Mo Cyclex has a tensile strength of 1030 N/mm^2, where Reynolds Mn-Mo 531 alloy is 802 N/mm^2. One might think this is only relevant in engineering testing, but stronger alloy can mean a lighter frame. With the stronger alloy, you get the same material durability with thinner walls. The springy properties of the two steels are the same, so the thinner wall could give a flexier frame.
However, Columbus made almost all the SL tubes at least as thick as those of 531C, at least based on late '80s info. So Columbus SL frames should be a bit stiffer than 531C frames, slightly heavier, and stronger by about 20%.
My only Columbus bike is a Mondonico assumed to be made of SL, and it IS stiff.