I'm not using FreeNAS or ZFS, but I did buy this same motherboard for my own NAS, which is using Gentoo Linux and btrfs. My sharing is being done with SMB and a little bit of nfs, not iSCSI. Although my needs were a bit different, I'll go ahead and describe my setup in case it helps.

I paired the q1900m with two SI-PEX40064 4-port PCIe 2.0 1x cards. These cards match the speed of the PCI slots. Ignore the bad reviews - those are generally from people who installed into 2x or 4x slots, connected SSDs, and then wondered why they only get 1x performance. In this case, the motherboard itself is 1x so there is no point to a faster card.

Cost was $55 + $33 + $33 ... so dirt cheap. I already had the RAM, case, and PS from my previous NAS build.

This gives me 10 hot-swap SATA ports and plenty of CPU power for a NAS. Yes, at first glance the 1x slots appear as a huge bottleneck (4 x 6 gbps SATA into a single 4 gbps slot!), but the reality is that if you are using spinning disks, especially 5400 RPM disks as are common for NAS use, you're fine.

My WD green disks do ~85 MB/s alone, and if I read from all 4 of them at the same time on this card I get ~320 MB/s, which is 4x a single drive so that looks good. Also 320 MB/s is reasonably within the 500 MB/s limit of the PCIe slot. Everything points to the drives being the limiting factor here, so no worries about the PCIe slot.

Overall, I'm quite happy with the setup. With 10 SATA ports, the 16x slot still unoccupied, and USB3, I have plenty of expansion options. There are currently 9 internal disks (in hot-swap bays) in the NAS and it is consuming 80 watts. I literally went through my junk pile and connected ALL the old junk hard drives I had lying around that were 100G or larger ... just because I can. :)