Real World differences depends on your use case. If youa re talking transferring a single large file then as I understand it, you can max out the 10Gbe interface. If you are talking lots of small files then you still may max out the interface but that depends on how fast the sending machine can grab the data. If youa re just hosting Plex and watching movies, I doubt you will see any difference at all. Keep in mind that you need 10Gbe connectivity for all your networking items in which you desire that type throughput. So if I created a 10Gbe connection between my main computer, a 10Gbe switch, and the FreeNAS, I would expect my backups to run significantly faster and any data transfers between my NAS and main computer would just be fast. My Roku on the otherhand is limited to 1Gbe (wired) and that link would be limited to that speed. WiFi would be limited to it's capability as well.
There are other forum threads here which discuss the 10Gbe interface and throughput, most end up favorable. The only real issues I've seen are when someone buys a non-supported NIC or the network connectivity wasn't proper, like trying to use a 1Gbe switch for example.
For me, while it would be cool to have 10Gbe in my home, I would not spend the money as I would never really benefit from it in a home environment. It takes 42 minutes to backup my main computer (263GB uncompressed, 176GB compressed), which is done at 2AM by automation.
As for the hard drive speed tests using dd, your drives will be fast enough to fill the pipe and you have enough RAM to support it properly.
I don't know if this information helps any. Again, use case makes a difference.
