Your Keystoker KAA2 is similar in design to the Harman, so I'll give you my 2 cents on how to improve the efficiency.
If the combustion air blower doesn't run continuously, it should, especially if your going to run it all summer when chimney draft is minimal. It will prevent outfires when the draft isn't sufficient to maintain a fire especially with very low burn rates. With that said, I think that combustion air needs to be proportional to the amount of heat being produced, you want plenty of air when you are trying to pull lots of BTU's, but less air when it's idling along. Excess air at idle, just dilutes the flue gasses and pushes heat up the chimney. I use a two speed fan, with a speed control, but a second smaller blower would work also, I don't like the idea of a throttle plate over the blower, it limits the maximum amount of air that is necessesary to get all the heat that the boiler is capable of.
I also believe that the on- off control system that is supplied with these boilers wastes fuel also. The guys at Coaltrol know this, their system drives the stoker in a time proportional manner based on how much heat is needed at the particuar moment. A good analogy is trying to maintain your car at 60mph while alternately stomping the pedal to the floor and then completely releasing it. Its worse with the coal boiler because they respond slowly, and then overshoot the setpoint temp. The only solution I know of for this is to use some form of programmable controller, or other electronic device that allows the fire to ramp up to meet the heat demand, rather then go full tilt, and then cut off. Modern PID temp controls will do this, but most don't have cycle times long enough for a coal stoker, since they are meant to control electric heaters.
Lastly, if your Keystoker doesn't have any insulation between the sheet metal jacket and the boiler itself, add some, it picked my water temperature up between 5 and 10 degrees.