Hi Nate,
We use Nexenta with vSphere 5.1, but with 10GbE hosts as well.
You probably should change the record size of your NFS volumes to 8 or 16 KB instead of 128KB if you need more IOPS instead of throughput. The VMware VMDK files are comparable to iSCSI volumes. If your VM's would also access NFS shares directly as well, you could/should set those shares back to 128KB.
For the SQL server you could create a separate NFS share/volumue with a 64KB record size for optimal performance. We use Oracle with Direct-NFS which is easier and faster because it uses the NFS shares directly, without a local VMware disk in between.
I would keep NFS as a datastore backing instead of iSCSI. NFS is much easier to setup and more space efficient. And I believe there is not much difference in performance.
Regards,
Dirk.