What you guys have to understand is that a Socket A Sempron is NOT a crippled Athlon. It's actually an original-design Athlon XP. Some newer Athlon XP's are "better" than the original Athlon XP's
Background:
The original Athlon XP chips ran on a 133mhz FSB and had 256K L2 cache. All Athlon XP's up to 2400+, some 2600+, and all 2700+ models have 256K cache and are based on Palomino (.18 micron), Thoroughbred-A (.13 micron) and Thoroughbred-B (.13 micron) cores.
Athlon XP's 2500+, some 2600+ and all 2800+ and up are Barton cores. Barton adds another 256K L2 cache, for a 512K total, and forces a 166 mhz or higher FSB (most 256K models are 133mhz FSB, except for the 2700+, which is a 256K/166mhz part, based on the T-bred B core).
Example:
Athlon XP 2100+ is a 1.73ghz chip (13x133mhz FSB), with 256K L2 cache. The latest revisions are using the Thoroughbred-B core.
Sempron 2500+ is a 1.75ghz chip (10.5x166mhz FSB), with 256K L2 cache. All Semprons use the Thoroughbred-B core.
Athlon XP 2500+ is a 1.83ghz chip (11x166mhz FSB), with 512K L2 cache. 512K L2 cache means it's the IMPROVED Barton core.
So, a Sempron is nothing more than an original Athlon XP with a raised FSB and an inflated PR rating. Since the Athlon XP debuted with 256K L2 cache, the design is optimized for 256K but can take advantage of more cache. So, a Sempron is only cut down in performance per PR rating unit, not in actual core performance.
As far as the PR rating goes, the idea is for the PR rating to compete against a Celeron's GHz speed, whereas Athlon XP's PR rating was aimed at the P4. So, since GHz aren't equal between chip families, neither are PR ratings.