The main question is whether you need or want sync writes.
For an SMB filer you normally do not use or want sync write due the negative performance impact. The ZFS filesystem itself remains always valid due CopyOnWrite and the extra security to protect the content of the rambased writecache on a crash is quite small.
This is different to a database use or when you use storage for VMs with older filesystems. A crash there can mean an inconsistent database ex when a financial transaction that consists from a remove money from one account and add it to another is only processed in its first part or on an older filesystem data is updated but not metadata resulting in a corrupt filesystem. To avoid this effects you definitely want and need sync write to protect the content of the writecache on a crash.
A ZFS pool from one Z2 vdev made from disks has around 100 raw iops (without help of caches). Your HGST SSDs are rated 57K / 25K IOPS Random Read /Write so they are at least 200x better regarding iops. So yes if you use sync write they will help to improve write performance up to 10x say for example from 30MB/s to 300 MB/s compared to a unsync sequential write performance of for example 1000 MB/s with some disks.
Another question would be if the HGST makes a good Slog?
The answer is definitely yes if you compare against other Flash based SSDs. It is made of fast SLC with powerloss protection. An Intel DC 3700 (mostly regarded as best Sata option) may be a little faster but they are both the upperclass Flash solutions. The HGST has the SAS advantage what allows multipath io and HA solutions.
Despite that, there is now a much faster solution with Intel Optane. Up ftom the 800P this will outperform any Flash solution. Only the powerloss aspect is different as Intel guatrantees this only for the top model 4800X. The lack of Dram caches makes the risk maybe tolerable with the smaller Optane.
See my performance sequence where your HGST may be a comparable with the best Sata SSD Flash.
http://openzfs.hfg-gmuend.de/napp-it_manuals/optane_slog_pool_performane.pdf