Jazz did you hear anything from Chai?
With 6 plastic inserts they must have 6 rows of balls, as each insert is a ball return tube. Additionally they're not going to use 3, or any odd number, rows of balls as that would result in uneven wear and all sorts due to having more balls running in one track...
I wonder if they're preloaded. A twin start screw with offset tracks in the nut is one way to apply preload - that would make them worthwhile.

As a general rule of thumb don't go below a 15T pulley with 5M HTD belts, and if you need much above 3:1 use double reduction. Use one of the many on-line calculators to find the number of teeth in mesh on the smallest pulley. They will give a warning if it is below 5...

http://www.sdp-si.com/cd/default.htm