If a ZFS dataset goes over 80% utilization, I understand it will switch from "first fit" to "best fit" which can permanently and severely impact performance.
https://serverfault.com/questions/733817/does-the-max-80-use...
https://serverfault.com/questions/733817/does-the-max-80-use...
It sped right back up again when I deleted some stuff.
I wouldn't worry about it.
On the side of ZFS, you can take one volume of a mirrorset, move it to another system and mount it in degraded mode, and copy a bunch of new files to it. When you return the updated drive to the full set and mount it, it will resilver the older drive with the newer content automatically.
That's a killer feature. With btrfs, you have to rebalance to mirror the new files, which takes a long time as every block must be rewritten between the mirrors. ZFS knows how to move just the new files in a resilver.