SQL Server MVP Aaron Bertran recently asked for elaboration on the comments posted here:
Specifically, "Performance improvements for SQL Server Service Pack 2 provides performance improvement for SQL Server 2005 under intensive workloads . These improvements are installed by default upon Service Pack 2 installation."
SQL Server PM Umachandar Jayachandran related that the improvements are related to winsock API. The change in the winsock API is more perceivable on SQL workloads running with 8 CPUs + multiple NICs. For example, you may notice it when running a high-end SAP load. It will not be noticeable/measurable on other configurations. Note that you'll need both Windows 2003 and SQL Server 2005 SP2 or higher also.
Thanks Aaron and Umachandar!