One possibility is to try changing rsize and wsize at the client end. Is the 10G network used for NFS the same network as the one used for the iSCSI? I've tried several values of rsize and wsize and it doesn't change anything. iSCSI and NFS are not on the same VLAN. I'm testing with servers which are on the same chassis (NFS/GFS servers are on blades and client is a KVM guest with virtio network card). iSCSI and NFS use separates network in the chassis but each blade has to share iSCSI (50% to 100% of 10 Gbits/s) and LAN (50% to 100%).iperf between server and client : 3.4 Gbits/s (435 Mbytes/s).You could also try using Jumbo Frames on the network.
I've tried but the NFS speed is the same.