Yes, I'm using ZFS.
I just did some testing with Plex, and it seems like two simultaneous 1080p transcodes is the limit. Tested with direct play on my Desktop, and forced transcoding on my Samsung Smart TV, Nexus 7, and Galaxy S4.
I tried many combinations, and these work:
1) Direct play on desktop + 1080p (quality level 10/12) stream on TV + 1080p (8 Mbps) stream on Nexus 7 or Galaxy S4.
2) Direct play on desktop + 1080p (quality level 10/12) stream on TV + 720p (4 Mbps) stream on Nexus 7 + 720p (4 Mbps) stream on Galaxy S4.
3) Direct play on desktop + direct play on TV + 1080p (8 Mbps) stream on Nexus 7 + 1080p (8 Mbps) stream on Galaxy S4.