I wish I had an answer for you, but I think at best I just have a similar problem and my workaround to share. I found my samba transfer speeds from NAS to core over 10/100 to be about 25 mbps, while replacing my core with a windows laptop as the samba client got me about 80 mbps. I did not bring the issue up here because when I booted an ubuntu live cd on the same laptop I saw the 25 mbps on that as well, and so chalked it up to the linux samba client implementation.
My workaround for most of the issue has been to convert my file server to NFS shares, which resolved my HD playback issues. However, I still have a samba-only NAS that I have been unable to get acceptable transfer speeds out of.