I can't believe that this matters to anyone using DSL or Cable. QoS reserves 20% of your POTENTIAL Network bandwith, NOT your Internet Bandwidth. My download speeds remained the same since I have upgraded from Win98SE to WinXP. This basically limits the speed your network card tells XP it's running at. Probably either 10mbps or 100mbps. It's reserving 20% of that, not 20% of your cable/DSL bandwidth. If you do get 10/100mbps downloads, you're very, very lucky.
I think Tuniss is right. I applied the tweak to disable the reserved 20% and did speed tests at DSL Reports before and after. The test were nearly identical.