HI develop -
Yes, support has been released and you should be able to see the option in the 16.3 release of the drivers available on www.intel.com. Unchecking the "reduce link speed during standby" will tell our LAN driver to allow for 100Mbps link when the system is in standby mode. You should see this in the "Power Management" tab.
I think S5 will be a BIOS/firmware fix by the OEM. Now its in the driver (at least from Intel) I might ping Dell to see if they can do anything.
I doubt 1Gb will be an option as mentioned earlier in the thread, there is only a certain amount of power available in S5 and linking at 1Gb might use too much. When you see how much it uses you can see why all these power saving options have been enabled/created.