You might try tuning the targets themselves:
1. From the console, press CTRL+ALT+W,
2. Select Tuning options -> iSCSI daemon options -> Target options,
3. Select the target in question,
4. change the values to the maximum required data size (check w/ the initiator to match in iscsi.conf file for linux).

maxRecvDataSegmentLen=262144
MaxBurstLength= 1048576
Maxxmitdatasegment=262144
FirstBurstLength=65536
DataDigest=None
maxoutstandingr2t=8
InitialR2T=No
ImmediateData=Yes
headerDigest=None
(deprecated)Wthreads=8

Also for best performance, attach only a single LUN per target.