Performance problems of iSCSI Box in Network

Hi Forum,
we use a iSCSI Box in our Network (received it on Monday) and did some testings. we experienced not that great performance boost using it and we do currently not know, where the problem could be.
here are some facts:
2x Xeon dc 2,0
4 GB RAM
Supermicro X7DBE
Adaptec ASR3805BL 8x - 6x Fujitsu 147 GB 15k RAID6 using 1 hotspare
Intel Pro/1000 PT Dualport PCI-E
Intel Pro/1000 onboard 2x

Gigabit LAN
iSCSI connected to Core Switch through 4 Gigabit Copper cables

Our Problem is:
When we connect from a client to the box we only do have about 4MB/s transferrate.
Connecting one client to another is about 12MB/s (clients hd´s are not that fast)
Clients also do have gigabit nic´s.
maybe we did some settings wrong on the box, or we need to do some more settings on the hba?
Thanks in advance for your help.

Try re-creating the Logical Volume without the default of initializing it and enable Write Back on the Target in Target manager. Check to make sure that all firmware on hardware is up to date. New release of iSCSI-R3 Enterprise will be available near the end of the month with new drivers so this might help your hardware. If these options do not help send in the log file for our engineers to review. Status > Hardware > Logs then click on Download.

Out of curiosity did you do a test with RAID 5 and what is the chunk size of the RAID 6 and cache? Anyway you can contact Adaptec to see how the performance of the RAID is doing?