QoS being pointed out at layer 3? That would be the router layer, not the application layer.
An Xbox will out perform a PC any day when it comes to gaming. The Xbox is a specific set of hardware and the games are tailor made for best performance on those devices. As with a PC and so many varying parts, you will see varying latency depending on your hardware, drivers, configuration, etc.
As far as what L-S-D was mentioning about QoS for MS products, he really was referring to BITS. BITS is used to update Microsoft products, not QoS. By default, QoS is disabled on a Windows workstation. BITS will use the maximum available bandwidth to download when available. If you start gaming, BITS can stop downloading if needed. Althought in most cases you're not maxing out your network connection playing a game, latency will become an issue when multiple threads are using your bandwidth.
Basically, sure, turning off BITS will help improve your gaming performance.. if you're in the middle of downloading an MS Update. Then again, if BITS is off, it'll use all available bandwidth to download. In that case, it'll be worse for a short time.
As far as QoS goes, it does nothing more than secure X amount of bandwidth for the specified item. If something requires 80% of the bandwitdh, it'll ensure it has that bandwidth. This could affect your gaming depending on your configuration. If you have multiple streams being throttled by QoS, the router may add latency. A cheaper router would most likely have an issue, whereas a higher grade router would be able to better process the traffic.