A ZFS Intent Log is only used for synchronous writes, and a couple months ago Storj nodes were switched to use async: so adding a ZIL won’t help. Eventually writes must make it to disk, and they tend to be small, so you may not to be able to reduce overall IO much… but you can still use a SSD with ZFS to speed up housekeeping tasks: like used-space-filewalker.
Quite a few here have used a ZFS “special metadata” device: so the millions of filenames/sizes can quickly be queried from SSD instead of hitting your HDD: which reduce most Storj internal tasks to seconds (instead of hours/days). I think 5GB/TB is a common config: so even small SSD partitions/devices make a difference. Search for more ZFS posts in the forum: Good Luck!