The Barton (remembering from reviews) is also clocked slightly slower, though it does justify the 2500+ rating.
Few things found huge benefit from the extra cache though
http://www.digital-daily.com/motherboard/amd-barton
Looks like a resonable comparison - and it seems the tweaks in the Barton (more than JUST cache) let them slaughter the clock speed for a similar performance rating - and reading between the lines, they clipped rather more off the clock speed than they should have done!
T-Breds at 166 FSB
So, AMD has added three new processor into the assortment: 2600+ (2083MHz), 2700+ (2167MHz) and 2800+ (2250MHz) which ran at FSB=166 MHz and featured 256K of L2 cache.
Bartons at 166 FSB
As a result, the performance of the core has been increased, which again allowed AMD to revise the processor rating. The following processors have been released: 2500+ (1833MHz), 2800+ (2083MHz) and 3000+ (2167MHz).