I don't have a Leisure Line stoker, but I do have a Reading Tri-burner converted with a full time blower. I've burnt buckwheat all winter long and went through 4 ton of it in the stokers this past season. Some of the buckwheat was pretty large. It looked like a mix of fines to small nut. The buckwheat coal did not burn all the way through. I had a good bit of unburnt coal in the ashes. I didn't see any increase in heat output vs. the rice sized coal. What I did notice is that I had to slow down the feed rate and increase the blower cfm. I replaced the 33 cfm blower with a 60 cfm blower. By slowing the feed rate and increasing the air flow through the buckwheat coal, I was able to burn through about 3/4 of the coal and give about the same heat output as the rice sized coal.
I did notice that the buckwheat sized coal burned rather well at a "no call for heat" or an idle fire, with nice blue flames, at around 50 cfm air flow through the grates. As the burn rate increased, because of the need for heat, is where I began to notice that the buckwheat sized coal, only the larger pea to small nut sized coal, did not completely burn through. It sure gave off some very large yellowish-orange tall flames though.
After experimenting with the different sized coal in the stokers, my conclusion is that a higher cfm airflow through the grates was more beneficial through all the sizes of coal. It was the feed rate that needed to be adjusted to the size of coal being burnt. One down side to the higher air flow was that the rice coal put out too much heat at an idle fire and if slowed down too low a feed rate would go out or too high an air flow would blow the coal right off the grate. The up side was being able to use much larger coal at a very low feed rate to give a nice constant low heat radiation in milder weather. The larger air flow pressurized the firebox and kept the draft better up the chimney in warmer weather.
As for burning buckwheat or larger coal at a higher rate, my conclusion is that if the coal feed rate could be adjusted to burn the coal more completely, slowing the feed rate and increasing the air flow even higher than the 60 cfm that I tried, I believe it would be a better match. I believe that one reason I did not notice any more heat was that the physical size or area of the stoves heat exchange flame path, was not large enough to take advantage of the larger coals heat output potential. Thus higher stack temperatures and wasted heat up the chimney. The other reason is that the size of the grate area of the stokers was limiting the heat potential of using the larger sized coal. I was trying to exceed the grate areas ability to burn the coal completely by feeding the coal faster than could physically be consumed.
So, I believe that there is a place and a happy middle ground mix for burning each size of coal with these over feed style stokers. I do use the Coal-trol on my stokers, so adjusting the feed rate was very easy to do. So now that the weather is warming up a bit, go ahead and try some buckwheat coal through your stove and see if you get similar results. Let me know.
Best experimenting, DOUG