Frankly, there is no payoff here. Read http://support.microsoft.com/kb/316666
"Correction of some incorrect claims about Windows XP QoS support
There have been claims in various published technical articles and newsgroup postings that Windows XP always reserves 20 percent of the available bandwidth for QoS. These claims are incorrect. The information in the "Clarification about QoS in end computers that are Running Windows XP" section correctly describes the behavior of Windows XP systems."