At higher burn rates the stove pipe is much hotter. (I can touch it at lower rates but not at higher ones). It should help mitigate the decline in efficiency from turning your stoker up and since the outside temp would be low, keeping a draft wouldn't be an issue. Also the thermostat should help keep a strong draft. For what its worth I called the manufacture of my stoker (Alaska) and they simply said not to use it. I was hoping to find someone that has used one to see if they had any problems as Alaska implied. I can't see how it would hurt anything, and how it couldn't improve efficiency when you're running it at higher rates (how much it would improve efficiency is the $20,000 question). But I wasn't willing to try it against their advice.
As far as whether or not its worth it, can it save enough coal to pay for it in 3-5 years? (They seem to cost around $150, losely the cost of a ton of coal). You might try to figure out the BTU/hr. heat loss based on the difference in pipe temperature. For example, is 350 degree pipe temp. vs. 130 degree pipe temp. indicative of a certain amount of BTU/hr. lost?
If it's 30,000 BTU/hr. (the max that any of these heat reclaimers claim), then it would take 833 hrs. or 34.72 days (25 million BTU/ton/30,000 BTU=833 hrs = 34.72 days) to save a ton of coal. So if you get 30,000 BTU out of the reclaimer for 35 days it pays for itself. The amount of days seem reaonable on the highest setting (enough to create that many BTUs) over 3-5 years, but I wonder what the pipe temp would have to be to get 30,000 BTU/hr. Is the 30,000 BTU/hr. based in reality or just a sales tactic? Is it more representive of a wood and not a coal fire?
Of course this doesn't factor in the cost of electricity to run the heat reclaimer!
From the convenience factor, if it allows you to run your stove on a lower rate on the coldest days and you don't have to add coal and remove ashes (or if it stops you from running out of coal) as often maybe it's worth it on that basis alone. Also you could get a little less coal each year.