Make sure the LUN has the Write Back enabled. Also check the test.log and tell me what the hdparm are for the reads - cut an past on your post.

Also SeanLeyne is correct that those drives are slow "Seagate 5900 RPM" with the seek times at that rate dont expect great performance values especially again a RAID controller with faster drives again slow drives with no RAID controller to tune with the Write Cache that is available with the RAID controller.