Any OS doing the serving should be able to send files at the max speed of the ethernet port it uses or something is configured wrong.
When I stated speed thats not what a transfer trails off too, just the speed I get after setting my hitachi drives for cooler operation, and running the cpu at 800mhz. These speeds are pretty much to or from drives set for power efficiency.
With my setup there will always be overhead to run samba and the softraid, I'm more than happy with my transfer speeds. (it' shouldn't, but I now want to try for some reason. ha.)
Anyway, you've motivated me to actually try the nic bonding I had planned for the server and see if this has any affect on throughput.
I can agree that group and permissions may be a bit confusing for some, but it's pretty easy to work out. There are lots of tutorials for FreeNAS and an active user community.
As someone with VERY limited Unix/Linux experience I can say that I learned a lot while setting it up, but overall it was pretty easy.