Are you sure you are focussing on the right bottleneck? I do not run an unraid system, but reach speeds of over 100MB/s over de default NIC without options like Jumbo frames and link doubling. I run OpenIndiana and ZFS on 3x 2TB WD EARS drives of 5400 rpm.
People using WHS have reported better results http://www.avforums.com/forums/networking-nas/1534144-hp-proliant-microserver-n40l-owners-thread-2.html.
So if you reach speeds of around 15 / 32 MB/s it seems to me that there are other factors you should look at before investing dollars.