I just wrote up my experiences on the little quest to make the trackball of my MNT Reform buttery smooth, including some embarrassing mistakes:
Hey, this is a really incredible hack, we are very impressed here and will try to replicate some of what you did. Also I’m working on outsourcing the production of the keycaps for the trackball, so as soon as we have more of them in stock we’ll send you replacements ^^
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Thanks for the feedback and the kind offer.
Out of curiosity, I continued experimenting for a bit. Just sharing what I found.
I tried installing 2.5 mm chromium-steel bearings as a potential alternative of the 1.5 mm ones I described in the article. This makes a very noticable difference - for the worse. 
It feels not bad at first, but when routinely using the larger bearings, any slight speckle on the surface of the ball becomes quite distracting. The diameter of the bearings has apparently a huge effect.
I also tried out these guys which are of a material harder than steel, thereby promising even less friction. However, even though the movement seems smoother, I found that the sliding of the ball over those bearings produces a hissing noise, prompting the disapproval of my family.
As a bottom line, I found the 1.5 mm chromium-steel variant most satisfying.
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Interesting, I just made my own version using 2.5mm bearings as they looked like a decent size. I’ve found them quite nice in my opinion, and haven’t had the issue of dust on the contact point yet. It definitely has a bit of a “scratchy” sound but I had assumed that was from the texture on the delrin ball and not the bearing itself.
Since I also ordered 1.5mm bearings at the same time I’ll have to print another model that takes them instead and see if it matches your experience of the 1.5mm being smoother.