I think the big change for those who have burned for years is that the new stove doesn't exhaust as much air as the old pre EPA units did. The guage may show a high temp, but that doesn't correlate to the number of BTU leaving the house. A new stove only pumps between 10 to 25 cfm up the flue (source
www.woodheat.org) - the old units exhausted way more. With less air leaving, the flue temp is higher (less dilution), but that doesn't correlate to lost heat. If the flue isn't hot, you won't get the draft needed, because you don't have the velocity in the flue.
I know that doesn't seem to agree with the experience that some folks have had, but it makes sense to me.
The other concern I have is that the EPA approval of the stove is based on the air being managed the way the stove was designed - when you add a key damper, you aren't running the unit the way it was designed (It wants the air to be restricted before the fire, not after, as this will affect the flow patterns in the firebox), and you won't necessarily get the clean burns that the design intends.
My 2 cents - Canadian at that, so multiply by 0.75 .