"You are correct -- I am in a niche area. I have less than 12 HDDs in a RAIDZ2 configuration. If I had more HDDs in the RAID I would see better write speeds."

"You are correct -- I am in a niche area. I have less than 12 HDDs in a RAIDZ2 configuration. If I had more HDDs in the RAID I would see better write speeds." That is incorrect. RaidZ(x) vdev write speed is equal to the write speed of a single drive in the vdev. If you want a raidZ zpool to have faster writes, you add a second vdev - that would double your write speed. A wider vdev, on the other hand (15 or 18 drives, etc.) would not have faster write speeds.