The big issue that doesn't seem to be mentioned at all here is the fact that the network settings for optimal gaming are different than for optimal office use (ie Windows default).
To quote from Leatrix:
"Online games generally use the TCP protocol which requires that network segments sent to your computer be acknowledged in order to provide a reliable connection.
Windows bundles these acknowledgements together and sends them in pairs. While this is an efficient way of dealing with them generally, the inevitable delays caused by the bundling process increase latency considerably.
This is because when Windows queues up an acknowledgement in order to bundle it with the following one, the game server has to wait for the acknowledgement timer to expire before sending new data.
Leatrix Latency Fix removes the acknowledgement bundling process so that an acknowledgement is sent immediately for every segment that's received. This produces a significant reduction in latency as there is no longer a delay before new data is sent to your computer.
In a normal networking environment, you would prioritise network efficiency over latency and use the Windows defaults, but in online games the opposite is true and you want the lowest latency you can possibly get."
There is a whole article there as well as a tool to really lower your ping times dramatically by changing how TCP behaves. The cost? The time it takes to move huge files goes up. I suppose there are other costs as well but most of us are after really low ping times. http://www.wowinterface.com/downloads/info13581-LeatrixLatencyFix.html