OK, I compared your settings with the default values in FreeNAS:
vfs.zfs.arc_max: 5074190464 [AUTOTUNE]
vfs.zfs.arc_max: 7217082368 (default)
kern.ipc.nmbclusters: 10000 [AUTOTUNE]
kern.ipc.nmbclusters: 50000 (default)
vfs.zfs.txg.write_limit_override: 1073741824
vfs.zfs.txg.write_limit_override: 0 (default)
kern.ipc.somaxconn: 384 [TCP LISTEN BUFFERS]
kern.ipc.somaxconn: 128 (default)
kern.ipc.nsfbufs: 150000 [THESE ARE BUFFERS FOR SENDFILE / CIFS]
kern.ipc.nsfbufs: 0 (default)
net.inet.tcp.inflight.enable: 1 [ONLY AFFECTS OUTGOING DATA, TCP BANDWIDTH DELAY]
net.inet.tcp.inflight.enable: 0 (default)
net.inet.tcp.delayed_ack: 0
net.inet.tcp.delayed_ack: 1 (default)
In my books those settings are all related to sysctl so they should be all added into Sysctls section. I don't know if it matters, but this is where I will place them if I decide to change any default values.
net.inet.tcp.sendbuf_max 2097152 Generated by autotune kern.ipc.maxsockbuf 2097152 Generated by autotune net.inet.tcp.recvbuf_max 2097152 Generated by autotune
I'm going to try the kern.ipc.nsfbufs setting now with AIO Off (don't say I don't listen to you). :)