On those few days that the heat output was not adequate, and the stoker ran at full capacity, was the fire on the grate all the way to the end, maybe with a 1/2" of dark ash at the end? If not you can increase the feed slightly to increase the area of burning coal.
As Dave [WNY] stated, adding more air will increase the heat, but probably cause clinker formation. Not something to have to deal with, or be surprised by. A clinker that doesn't fall off the grate, blocking the fire up behind it,, well it will make a mess, possibly cause an out-fire, or cause a back up of the fire too high on the grate.. none of these are what you want to experience.
On those really cold days, did your barometric damper work adequately? Did it keep the draft in the flue under control? A lot of heat, coupled with a very cold temperature will cause your chimney to have very strong draft.. if the draft is too strong, it will pull a lot of the heat out of the stove, dropping the stove surface temps, lessening the heat available to heat the house.. I'm assuming you have a barometric damper..
Did you monitor the surface temps of the stove and flue pipe?? You need real numbers if you decide to experiment.. otherwise the variables of weather conditions will totally confuse any possible results, and make the experiments just guesses as to whether they are an improvement or not.
Hope this helps.