They are 9mm direct replacements but imo have a small window for proper tension. There are members that did quite a bit of testing on steel vs kevlar in this thread.
https://community.carbide3d.com/t/belt-stretch-and-stepper-holding-measured/18480
I can see an xxl having a tougher time with belt stretch vs a S3 but there really shouldn’t be any backlash. Just dont tighten enough to the point you break the stepper shaft lol.