9mm belts aren’t stretching much.
This is the difference between 18 steps per inch: 0.000011"
Not an issue 
EDIT: and compounded over 39" = 0.00044"
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I’m so sorry for bumping this post.
This is a question, I guess for Erik. I recently did a round of upgrades to my 1000mm X-Carve with the X-Controller
(Background) I upgraded the 20x20 to 20x40 extrusions and added additional center bracing, I made 4" wide stiffners from 1/8" thick aircraft aluminum and stiffned the Y-axis extrusions to the frame, I redid the Y-Axis extrusion mounts using 1/4"thick aircraft aluminum and added a second row of mounting holes (6 on the horizontal and 2 for the extrusion on the vertical), changed the stock (140oz/in) steppers to 269 oz/in with 9mm belting and pulleys.
(Question) I would like to upgrade my trusted Dewalt 611 to a Bosch 1617EVS 2.25 HP Electronic Router. and I would like to modify the frame to extend the Y-Axis rails to be able to cut 48". That way I rip a 48"x96"sheet of material into 3, as opposed to presently ripping three sets of 31x32 and 32x17.
Do you see the need for anyother upgrade/modification?