About three weeks ago, I was casually browsing eBay and found this little gem, a Host Bus Adapter that can do PCIe 2.0 x8 (~4 to 8GB/s). This is way better than the one that I purchased earlier (GLOTRENDS SA3116J PCIe SATA Adapter Card) which can operate on a single lane of PCIe 3.0 yielding only 1GB/s. I could not pass it up at a price of only $ 40 CAD, so I purchased two of these to replace the old adapter card I had.
This new card LSI 6Gbps SAS HBA 9200-8i only supports 8 SATA ports per card, so I had to get two of them to support all of the hard drives that I have. These SAS HBA cards must have the IT (initiator target) mode firmware because the default firmware (IR mode) supports a version of hardware RAID, which I did not want. With the IT mode, the hard drives will be logically separated on the card and only share the physical bandwidth of the PCIe bus. This is a must for ZFS.
With these new cards, my write throughput to my NAS hard drives now averages around 500MB/s. Previously, I was only getting about half of this.
I wish I would have found these sooner. Now I have two spare PCIe SATA expansion cards, one supporting 8 ports, and the other supporting 16 ports. I will place them on another server. Perhaps in a future Proxmox cluster project.