I have an XL.
Ok, I took a stab at measuring belt stretch independent of the rest of my machine. I took each belt and clamped one end to the wall, letting the other hang down. I hung about 38 lbs of weight at 1000mm down the belt, and measured how far the 500mm point on the belt pulled down vs lightly hand-tight pulling the belt straight.
This should give us the stretch of a .5m belt segment with 38 lbs of force applied:
9mm stock belt: 2.9mm (.58%)
9mm steel belt: 1.5mm (.3%)
10mm steel belt: 1.3mm (.26%)
Those numbers seem to track pretty well with the measurements on my machine.
It’s very easy to see the stretch for yourself. Turn on the SO3, so the steppers start holding position. Push firmly against the carriage. You’ll probably see the carriage move a bit, the V-wheels will turn a bit, the belt will tension and loosen on the two sides… and the stepper pulley (and segment of the belt engaged with it) shouldn’t move.
Edit - correcting a number in the table, should be 38 lbs, not 19.