I would say that NFS IOPS are generally not superior to iSCSI, but because of thin provisioning and caching, it can equal or sometime outperform iSCSI because of the thin provisioning aspect. I've done extensive head-to-head comparisons of iSCSI vs. NFS. On modern servers, I will acknowledge that NFS can equal or sometimes exceed iSCSi performance, but so much depends on the whole setup.