this is a server I've got from eBay: https://www.ebay.co.uk/itm/322978590993
I3 3220, 8gb DDR3 ram, a raid card, 4 HDD of 2TB in raid 5 (card cannot do raid 6)
raid is set up from raid card and not from DSM.
I've followed this tutorial
I've installed DS3615xs 6.1 Jun's Mod.
yesterday I had installed on it freenas11 and tested transfer and got a constantly 80MB/s transfer (actually 113MB/s when sending the data from my PC's SSD hard) of the exact same 7.5 GB file.