The SF-250 has an optional firebox reducer. I have not seen a drawing nor an image but I did speak with the dealer's kid. To use it, you would remove 2 of 5 grates drop in a metal adapter plate with a wall to support fire brick. Essentially you reduce the output 40%.
I wrote yesterday, " Is there was a way to burn at a lower rate? " I began planning a design in my own mind using a steel plate to block the draft in a section of the fire box sized just right with a handle welded to it for quick removal if the stove was warm. Today I read some Harman literature and discovered Harman already had a design . It costs $75. Harman's design would require a shutdown of the stove.
So far, the heat off of the 250 has been good, maybe a little warm. But I made a commitment to coal last year as most listers on this forum. So I wanted to go with a unit that wouldn't fail no matter how cold it gets. And I find maintaing a coal fire sort of a hobby, not so much a chore. This is why I prefer a handfired unit over a stoker. So, yes, I agree, the 250 may be a little overkill but I found the Mark III just a little undersized to heat 2500 sf plus a full basement. By the way, I have listed my old unit on Ebay. I had 10 bids in less than 24 hrs.