Not really. Last I heard doesn’t work in the GUI, and was always there in the background.
EDIT: oops. Jumping the gun!
Asides from that, I really don’t think there is much point buying more 2 or 4TB disks. They’re uneconomical, so if one fails you’d want to replace with something larger anyway.
Mirrors are very flexible. Upgrade in pairs. Remove a mirror, add a mirror, etc.
And Raidz expansion is not without its own issues.