Is doubling RAM from 8GB to 16GB worth it for FreeNAS and ZFS ARC?

Yes, going from 8GB to 16GB will give ZFS’s ARC noticeably more room to breathe. 8GB is the bare minimum for FreeNAS even without heavy use — with ZFS, most of that RAM gets eaten by the ARC by default. Adding another 8GB essentially doubles your cache for recently accessed data, which means lower latency for repeated reads and less disk thrashing. It’s one of the best bang-for-buck upgrades you can make on a low-RAM system.