There is an option in Realtek PCIe GBE Family Controller, Auto Disable Gigabit, to save energy. If enabled, your system will detect when the network is idle and turn off ethernet functionality.

At this year's Computex, Realtek announced that samples of its RTS5781DL controller with a PCIe 5.0 x4 interface are now shipping to interested parties, so expect actual drives within the next year.