I must say the speeds dissapoint me somewhat, especially the read speeds. Normal HD's can easily reach 90 MB/S nowadays, so I have no idea why unRAID isn't capable of providing at least 90% of that speed. Since you're using pretty large drives, I don't suppose your drives are particularly old/slow as well. Are you using a low power (but also low performance) CPU like an Intel Atom or something?

Maybe FreeNAS with its ZFS file system is also a good option. I'll have to look into that.