Per @Microwave_Monkey’s request, I tested 5 more belts!
Procedure
Same as mentioned prior: Belt Stretch and Stepper Holding Measured
Results
Similar to what @LiamN saw, the Gates GT3 belts perform better than the GT2 fiberglass belts. Steel belts are still the best in terms of stiffness, as to be expected, with the fiberglass belts most likely winning in longevity. 10mm fiberglass belts are about 11% better than 9mm belts (as 10mm is 11% wider than 9mm), however, 10mm steel-cored belts are about 19% better than 9mm steel-cored belts. Perhaps the extra 1mm allows for a disproportionate increase in steel.