Just did the same on FreeNAS 11.
Jumped on TR's finding of 4x4TB HGST for $400 a couple weeks ago. Now running a 6x4TB RAIDZ2 for ~15TB usable. R/W exceeds 1GbE links.
Install was stupid simple, as I bought a pre-flashed IT mode card. FreeNAS saw it immediately.
All in all, happy with the results.