The socket-A Sempron is the exact same piece of hardware as Athlon-XP. They're Tbred-B, Thorton and Barton core, nothing changed except for the bus frequency and multiplier configurations - and the brand name. Manufacturing, packaging, core voltage, all the same.
So if you had an Athlon-XP 2400+ "Thorton" and compared it to a Sempron 2800+ "Thorton", all the difference there is is the FSB frequency, 166 for the Sempron and 133 for the Athlon-XP.