Hi Phil,
Given that you are seeing this with NFS-CIFS, do you see the same issue with HTTP and/or FTP?
Secondly do you have your NIC's bonded?

Thirdly, how are you getting such good write speeds? The reason I ask is that we have had major issues with the NICS in our SSR212MC <10MBs so we had put this down to an issue with the Intel Pro1000 driver, which was fixed by a mini update and broke again in the latest full update.....

Rgds Ben