Bit behind here but possible solution:
I have one of these devices and I get transfer speeds on large files of between 40-50 MB/s
I was messing around with this NAS (relocating it to another room) All seemed fine, transferred some files... only getting 9-10 MB/s
Checked in the web server on settings and noticed that there was only 100mbit option in the "LINK SPEED" setting.
Wierd... found out it was the LAN CABLE... used some crappy one that came with a cheap network device, switched it back to a decent CAT5e cable (homemade) and hey presto 1000mbit was back in the "LINK SPEED"
Transferred a 23 Gig Movie file at 47MB/s
Thats my 2 cents worth (Im from the UK BTW)