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Realtek's $10 tiny 10GbE network adapter is coming to motherboards later this year
(www.tomshardware.com)
This is a most excellent place for technology news and articles.
My gigabit connection is good enough for my NAS, as the read speeds on the hard drive itself tend to be limited to about a gigabit/s anyway. But I could see some kind of SSD NAS benefiting from a faster LAN connection.
If you shell money out for a full-flash NAS, you have the money to buy the bandwidth easily by the truckload.