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?