Yep, scrub is definitely needed.
I am running 20Gb (10Gb x2) connection with some (distant) plans to upgrade to 56Gb, but I think it will require disk array upgrade as well. Besides TrueNAS I have Proxmox running on the same server and using ZFS natively for VM volumes + a few other Proxmox servers accessing NAS through iSCSI and NFS + a few clients with a little occasional SMB workload. I also have pretty loaded PostgreSQL running on the TrueNAS machine.
Just tested with the second wire connected to the extension controller mode and got ~4-8% performance improvement (~1.3GB/s), which most likely is just a fluctuation.
It’s a homelab (used for my business needs though), so pushing everything to the limet is still fun even when there is no real need.