Quick update, not really anything groundbreaking. Upgraded memory to 4x 4GB DDR3 ECC from the stock 1x 4GB and migrated from the 500 GB drive to four HGST Ultrastar 7K4000 4TB.
I managed to migrate the array live thanks to the Synolgoy Hybrid RAID. Since I only have room for four drives, I added 3 of the drives, expanded the array, swapped the 500 GB for the last 4TB drive, and rebuilt it. Testing with hdparm, /dev/mapper/cachedev_0 is seeing ~472.49 MB/s reads with each individual drive seeing about 160 MB/s reads.
Following this guide, I was seeing a little over 370-390 MB/s writes to the array. I'm pretty pleased with that.