Not a whole lot more tweaking you can do really on the boiler now. Your will be chasing about 8,000 BTU/Hr. Keep in mind this is all seat of the pants figuring.
Assume 75% efficient burn based on their lab rating of the boiler at 150K times a realistic 75% efficiency gives you a real world rating of around 112K possible.
So if you are burning 200# and averaging 104K to a possible max of 112K that is pretty darn good!!!
You are heating a LOT of space. You might get a little bit more efficiency out of the buck but remember coal is coal is coal. No matter the shape or the size it has the same BTU per pound. It's not like wood where the BTU per pound value can vary a lot by species.
Keep in mind that Return On Investment in not only money but time spent chasing that illusive 8K BTU/Hr could be considerable.
Most of the folks on this forum have a backup system for the stupid cold times and we use it too!!!! You can heat everything comfortable except for that possible week a year where you put on the propane. Your sitting pretty good now from what I can
see!!! I have an oil burner in the basement for hot water heating and domestic water. However we heat the house most of the time with my Vigilant Castings 2310 stove in the dining room. The oil backup is there for times when I get sick for a week or so and can't tend the stove... which has happened the last three winters. Right now we are on the little bucket a day coal fired water heater which is piped so it can heat the house and make all our domestic hot water.
To quote a TV series line with a little modification.....You've done good grasshoppaa