WVM, I checked out your rig construction pics, and the thought that comes to mind is the higher stack temps to get it really going well. Is this a symptom of the slope back of the fire box transitioning lower than optimal for the drop flues? An idea that comes to mind is placing a row of fire bricks flat across the area at the transition just aft of the AOF tubes. This might function like a "trip strip" on an airplanes wing. Perhaps think nascar and wind tunnel testing, but in this case it's inside of the firebox. Might help mix the burning gases better, and get them higher up into the flues. The flames might go up over, and back down though, so it might boil even harder in the front of the drop flue pan, and behave similar aft. 5 gal/ hr improvement?