I've got it formatted as RaidZ (ZFS's equivalent of raid5) and have no problem hitting 80MB/second over gigE with SMB. Now, I've not yet tested iscsi on it.
I'm interested to know how heavy ZFS usage holds up with only 8GB of kernel memory. Is FreeNAS up to kernel 9.x yet? I haven't followed FreeNAS for a while. My smallest RAM machine with ZFS has 16GB.... and I am *loving* it. I wish it was easier to make it the boot default file system too!