I don't even think it's debatable about whether hardy's are efficient. The whole design doesn't tend on the side of efficiency, when you see flames coming out the top all the time, here is your sign.
There are many hardy's in my area, nearly everyone was installed poorly and folks burn 15-20 truck loads per winter on the average home. I've replaced several with the heatmaster 3000, folks said it cut there wood use almost in half, some of that was due to the stove itself, but a lot was due to a proper installation and not using frost king insulation inside schedule 40 lol
The stack temps out of central boiler and empyre would be about the same, same design onthe conventional stoves, heatmaster stack temps would be very similar unless it was an e model. The hardy with a hole at the highest point and very little to slow heat loss tends to have crazy high stack temps