Is the newest open-e version on it. They updated the Intel drivers, nearly every update. In one update the last month the performance increased with Intel Network cards.
I have similar issue, getting 200Mbits a second which is about 25MBytes per second. I expect a lot more using raid6 with 8 drives and BBU unit attached. I have Write cache turned on and all settings in 3ware card seem correct.
I have only now noticed that open-e has an option for iscsi for WB cache, should this be turned on rather than write-through which is the default?