There is an old iSCSI Design Considerations and Deployment Guide back from VMware Infrastructure 3 speaking of performance increase up to 30%: http://www.vmware.com/files/pdf/iSCSI_design_deploy.pdf
Also take a look at the vSphere 5.1 Storage guide: http://pubs.vmware.com/vsphere-51/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-511-storage-guide.pdf
There are about 80 pages about iSCSI, including best practices on how to optimize performance.