I have seen speeds vary from 50MB /s to 80MB /s, with DSS version 1.18 from the Demo-CD you can enable Write Back for your iSCSI LUN. It should improve iSCSI performance.
Also try changing the in console CTRL. + ALT. + W then iSCSI Daemon options to increase MaxRecvDataSegmentLength to 65526 and MaxXmitDataSegmentLength to 65536