I had the same thoughts with my IronFireman underfeed stoker. It is pretty much the same as an EFM stoker. I have an EFM stoker unit as well,[no boiler], so I have put them side by side for comparision.
What I found was that if you use a slightly higher feed rate, and let the coal burn without the combustion fan forcing air through the coal bed, you get a bit less unburnt coal and a bit finer ash. If your chimney has plenty of draft, it will pull air through the coal bed and burn pretty well.
Plus, if you have the rate setting low, there is no added margin to increase BTU output if it gets really cold and windy and your heat requirements increase drasticly.
When I had my boiler set near maximum, the outer 1-2" perimeter of coal on the burn pot was still glowing red under the surface, but once it reached the edge and fell off into the ashpan it went cold/black right away. Takeing a sample from the pan showed the ash to be very well burnt, with little unburnt coal.
I tried rice and buckwheat coal, I liked the increased heat output from the buckwheat, but it had a bit more unburnt coal in the ash. This could have been from the poorer quality coal, or it may have needed more time to burn all the way through the larger pieces of coal. I also burn some 'reclaimed fines' which were probably rice and barley size pieces. It didn't let as much air up through the coal bed so the heat output was lower than rice or buck. It piled up to a much deeper coal bed, maybe 6" deep instead of 3-4" for buck/rice.
I'd set your feed to a setting that had the stoker run at about 50%. Approximately 15 min. on then 15 min off. This will give you plenty of burn time from natural draft, and provide extra feed available for colder weather.
However: half the fun of getting a new stove or boiler is to learn it's characteristics. So I'd suggest trying several settings with different weather and see what works best for your setup.
There was a new member who had just installed an EFM furnace in a shop and was inquiring about setting the feed rate, I think he was burning buckwheat coal, and had a continous setting [no thermostat] of 5-7#/hr.
He was very happy with the heat output.