I have ten 1tb RE4 WD's in a raid 10 (~4.5 tb usable). Using a sata DOM for open-e OS. Seems a waste to use 2 drive slots just for the OS. I am using the 9260 LSI controller as well. I do have CacheCade 2.0, but I don't think that makes any difference because I was getting decent times with that that same test string even before I added the Cachecade. Cachecade really helps in speeding up all the websites so they are snappy and respond quickly.
Please check the following PDF at : http://www.open-e.com/download/libra..._file/get/131/
that will show you steps how to install DSSv6 on USB/DOM or storage LUN, as you can install DSSv6 on your storage disks, by creating a LUN with size of 2GB, then install DSSv6 on that LUN, in this way you can get red of the DOM/USB flash memory and save more slots.
Also we recommend you to use the latest builds of DSSv6 as it contain the latest drivers for your hardware.