Thanks a lot, I spent a ton of time spinning my wheels on this problem! I added a Dell PERC H310 SAS card to my setup along with 6x 3TB SAS drives. The parity sync was crawling along around 40 MB/s. Turns out all the write cache's were disabled for the SAS drives (enabled for my SATA drives on the separate on-board SATA controller). Enabled the write cache on the SAS drives and I'm at 140 MB/s now which is perfectly acceptable.
Some other stuff if anyone else bought an H310 and stumbles onto this. The H310 requires some work to get going (apparently well known). Had to tape pins B5 and B6 on the card itself so my PC would boot. Initially after doing that unRaid noticed all the SAS drives but parity sync was around 10 MB/s. That's when I stumbled onto flashing the H310 firmware with the IT firmware (I had the RAID firmware). That's what brought me up to 40 MB/s. The write caching was the last piece of the puzzle here. Link to the firmware bit.