For the time being, I have been pretty happy with the copper sandwich which now surrounds the 40mm, 6 amp TEC I have been using.  Hoping for a {more or less} apples-to-apples test, I mounted the copper-clad TEC to the P521 for a comparison of how the smaller brother of the P125 compares to it.  

Not surprisingly, the difference between these two high performance sinks doesn't really show up on the low stress boot up test.  With a starting temperature almost five degrees higher, the P125 sink reaches only two degrees higher on boot up.  The slug temperature is almost identical.  At the same room temperature, I would guess that the P521 sink would run about 3°F higher.

For having 55% of the mass and much less surface area of the P125, the P521 certainly performs well.  Judging by the results of these tests, at the same room temperature, the P521 would run only about 6 to 7°F over the P125.  (Too bad the weather didn't cooperate so I could run the tests in the same room temp.)  These results reinforce my views on the pin-type sinks being the way to go for optimal cooling.

Thoughts
For use without a peltier, and where space is at a premium, I haven't run into a situation where the Alpha P521 wouldn't be all the heat sink one would need.  If using one or more TECs is in your plans, the P125 is still the sink to beat. (For the time being....)