Jim a insert without a fan will still be much more efficient than a open masonry fireplace. A open masonry fireplace is moving air out the flue at 200-600cfm. It is literally uncontrolled combustion, being fed copious amounts of your homes air. For most masonry fireplaces, espescially exterior ones, but interior ones too, there is a overall net loss of heat for the home. The room it's in will get warm, the masonry will absorb heat and give some of that back(or transfer it outdoors), but the far reaches of the home will be effectively cooled.
A modern EPA insert will reburn its own exhaust, not only burning much much cleaner but creating much more heat by burning off the nasty in the firebox as fuel, instead of belching them out as smoke and pollution.
Burning without re-burn technology is a fools errand. Such a huge waste of fuel. And pollution to boot.