If done well I would expect a rocket mass heater to approach or be similar to the performance of a good masonry heater. But there is no magic here. There are only so many BTUs in a cordwood log. If the place can be heated with a few logs a day that means that either the area heated is small, or well insulated and/or the differential between room temp and outside temp is low. One can not achieve greater than 100% efficiency burning wood, which is what some claims suggest. 90% is pretty darn respectable if that can be achieved.