If your filesystem is populated with more than 80%, then the performance can be drastically decrease. Please try to increase filesystem size to become less that 80%. Several options are available with zfs command to force ZFS filesystem size or to have an minimal size.
Utilization isn't a big deal as long as your average service times don't go up and you don't have a lot of requests sitting in queue. I think this is the best part of zfs, that you can keep that many disks consistently busy.