I am aware that hand firing will be more efficient than a stoker but I was not aware that stoker boiler efficiencies would be on the order of 50%.
The fuel cost calculator at hearth.com has a table of likely efficiencies for various fuels and burn methods with modern stokers listed at 70-85% and older boilers in the range of 60-75%. Axeman-Anderson's Anthratube boiler boasts an efficiency of 75% in their online literature. They claim that from 2000lbs of anthracite, 1500lbs will produce heat while 300lbs will be ash and the rest (200lbs) goes out the flue as waste heat. They imply that the efficiencies for other makers of boilers is on the order of 45-50%.
After talking with a sales contact at that firm today, I realized that their design, while very sound, is at least 25 years old. The best coal could do for efficiency would theoretically be total burning down to its ash content or about 87% to 91% from information found online. Since we know that stokers are less efficient than hand firing, we could assume this efficiency to be lower in practice, as you stated. But, is it likely that the AAA achieves 75% with an ash content of 15%.
Anyone out there have one?
As for the ashes, I have found a clean fill pit that I can dump them in without charge.