@Ender117 and @HoneyBadger - Moving discussion to a new thread.
I was maxing out at 4G NFS writes with my pool constructed as follows:
root@freenas2:/nonexistent # zpool list -v RAIDZ2-I
NAME SIZE ALLOC FREE EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT
RAIDZ2-I 14.5T 5.75T...