Sounds like your doing read and writes in random mode. Depending how much money you would like to spend on top end equipment you should be able to see better rates.
Especially when bonding nic's at 802.3ad, having a fast processor with a lot of cache. A top end controller with a RAID set to the highest chunk size should help as well. Some of the SuperMicro and Intel mother boards with fast RAM and up to date firmwares. Also we are focusing on our new iSCSI R-3 Enterprise which is based on USB DOM technology - here you will have improvements as well. Remeber when running test make sure you do r / w with random and sequentials when testing to verify with 512B , 1K ,2K,4K,8K,16K,32K,64K,128K,256K,512K,1024K,2048K,4 096K.