Have you ever tried to actually get gigabit speeds from the crap hardware installed in most PCs? Most companies use low end realtek or marvell controllers which may, if your lucky, have a good switch and have jumbo frames enabled get you to ~650-700 Mbps on a single large continuous stream. Performance only goes down from there are multiple connections are dealt with. If you want to take a look for yourself download iperf and see what you can actually put through standard consumer NICs and switches. The only NIC controllers I've seen actually get to true gigabit speeds are from Intel, and you still need a decent switch.