Hello,
I assume you're writing to a ZFS pool? If so, I have a fix for you. If it's UFS, this may still work, but it's untested.
FreeNAS is using FreeBSD, and the way I fixed slow NFS speeds in ESXi was by patching the NFS code to ignore O_SYNC commands from ESXi. Basically I'm making it more like iSCSI for speed, but with the ease of NFS.
Check out : Technical Musings: Speeding up FreeBSD's NFS on ZFS for ESX clients
That patch still applies for FreeBSD 10. I'm currently running through benchmarks in FreeBSD 10, and I'm eying cutting out the NFS cache subsystem as it's unnecessary duplicated overhead when you're working with a ZFS pool. Not sure how many people use FreeBSD/FreeNAS as their ESXi SAN technology, but there is a lot of performance to be had with the right code.
I've run 10+ machines in production for years with the patch, and have yet to experience any corruption issues.