The point has been made that a stove is more efficient than an insert because it's got six sides exposed to the room from which to radiate heat instead of just the front and top as an insert does.
I've got both, an old airtight insert and a freestanding stove; when I moved into this place (rental) 15 years ago, I knew the insert would never keep up(~2000 sq ft, no insulation, drafty doors and windows) so I had an old airtight stove installed.
About three years ago I replaced the old stove with a PE Summit. I was really concerned after the first year (installed the stove in March, so only a little burning time left) that it would not keep up with warming the house; it just didn't get hot like my old airtight stove. Then I installed a blower onto the PE stove and never looked back. I realised, basically the top of the stove is the only part that really gets hot in a modern EPA unit, so blowing this constant curtain of air across the hot top creates actually an enormous volume of hot, moving air in the house, hence the entire house is much warmer now with my new EPA stove than with my old airtight.
Considering this fact that the bulk of the heating from my stove is created by blowing air across the hot top, I have to believe that an insert of similar design (complete with blower) would be nearly as efficient at heating my house as the new stove is. It will still function if the power goes out, but less effectively. Also not to be discounted is the considerable warming of the room the stove is in from radiant heat from the glass door (if you get a glass door). Even though it takes the stove a long time to get hot, if the house is cold you can sit in front of the fire as it gets going and be warmed by the flames throuhg the door.
If your fireplace is big enough to facilitate a realistic sized insert, and you can deal with a blower to move air across the top of the thing, and assuming that the support for the insert (flue, etc) is in good condition, you very well might see similar heat from an insert as from a modern free standing stove.
Regards EPA stoves, it is very satisfying to go outside and see only heat waves rising form your pipe, virtually no smoke.