Samba being single threaded has been completely blown out of proportions.
First off, Samba is only single-threaded per connection. So, if you have ten different people connecting, each connection will be its own thread. You're not bottlenecking everything through one core.
Second, only the data transfer component is part of Samba. The actual data processing (retrieval, writing de/encrypting, etc) is part of FreeNAS, and will use other threads. For any modern processor, including an Avoton, the single threaded Samba performance is more than enough to saturate a 1Gb ethernet link. With unlimited network bandwidth, a modern Pentium would be more than capable of serving the max speed of a typical 6-disk RAIDZ2 array, in which case your array, not your processor, would be your limiting factor.
And lastly, even if you got to a place where your single-threaded Samba performance was your bottleneck, I'd wager that your CPU is now seriously undersized for the rest of your system, and you'll have bigger problems than Samba.