I believe the performance issue is on the Xenserver side. I have a DSS V6 and sharing it with a XEN network and Xenserver 6 network.

All servers using MPIO (2 1G cards) and the SAN has a 10G card. (using Block i/o)

using your test:
dd if=/dev/zero of=/var/tmpMnt bs=1024 count=12000000

On a linux server under XEN I was getting about 85MB/s (we have SATA drives in DSS) and on a linux server under Xenserver 6.0 I was only getting about 28MB/s

All settings on DSS had been optimized as per various posts in this forum for each volume. See various posts about ISCSI performance when goggling 'iscsi performance xenserver'