Although Runaway Pancake is quite right, this post has about 12K views (including me) from people that, I suppose, seek the best way to power their arduino from a 24V source.
I don't think that fritz_dan is still looking for an answer but I found the subject more than helpful so.. thank you guys for posting your thoughts after 3 years!
Back to the subject!
I think arduinoaleman has a point. There is no need for a 95% power efficient solution when powering from a wall socket and draws less than 100mA. Even at 200mA the wasted power is not a critical loss for the critical time of our planet.
Cant we split the heat dissipation with 2 regulators like 7805 and 7812 in order not even to use a heatsink?