The first of the fistfull of things I bought on eBay last month has started arriving - I'm addicted to buying $5 shit from Hong Kong, but not addicted to waiting the time it takes for surface mail shit to arrive.
One of the first things I bought was a Realtek Gigabit NIC (a PCIE RTL8111C) for the desktop, which is plugged in to the TV and Duncan uses it. It looks like the on-board NIC in it wasn't Gig-E, which is why I was getting such shit speeds on it. I wasn't expecting much out of a $4 (shipped!) Realtek NIC, as I've had pretty bad experiences with shit NICs before dropping packets and so on, but this one actually works rather well.
iperf3 results look promising:
[ ID] Interval Transfer Bandwidth [ 4] 0.00-10.00 sec 1.07 GBytes 920 Mbits/sec sender [ 4] 0.00-10.00 sec 1.07 GBytes 920 Mbits/sec receiver
I'm considering putting a Gigabit switch over near the TV and wiring the PS4 to it as well, so when we're streaming media to it, the Wifi isn't being saturated.