Since I changed the belt clamps, my steel core belts were too short to put back on the machine so I installed some Gates LL2MR09 glass core belts. 

I originally chose steel core belts for the hoped-for minimal stretch, though there wasn't much information about them available.  I measured the displacement of the bed surface with 4 kg piled on it,  It worked out to 42 um per kg of print load, equivalent to a maximum of 1.2 um in any single layer of a print that covers the entire bed.  

Yesterday I checked the glass core belt stretch.  It was about 3.4 x greater than the steel core belt- about 150 um/kg equivalent to a maximum of 4 um in any single layer of a print that completely covers the bed.  Probably still not an issue for most prints.

More:  https://drmrehorst.blogspot.com/2018/12/comparing-steel-core-and-glass-core.html