Well, I transferred some MKV's over last night. Had to say that it was painfully slow. For 42GB, Windows was quoting around 7 hours, it was shifting at approx 1.9 MB/s. This was from my Windows 7 laptop, wireless to the router, wired thereafter. I got a marginal improvement going completely wired, but even that was only 2 MB/s. I have set the NAS as RAID 1, ext3, using 2 brand new Samsung F4 2TB HDD's.
I'll try some detective work and see if there is a problem elsewhere.