Though my Expert (wired) trackball is working fine, I wanted to try the silicon nitride ball bearings based on their reputation online. Sure enough, they make a significant difference in smoothness feel and glide. It's not for everyone, but if you are a neurotic i'd recommend it.
I've replaced bearings with zirconium oxide for my Elecom Huge and Deft Pro and found that the difference was negligible and non-noticeable. I did replace the bad scratchy bearings on my Elecom EX-G Pro and the zirconium bearings made a world of difference. It shows that it's a luck of the draw thing, but if your trackballs are harsh and gritty feeling, replace the bearings with anything. But I can tell you that subjectively these silicon nitride bearings are at least 50% smoother and glide more freely than the zirconium ones. Probably it's because silicon ceramic balls are just more slippery than zirconium.