one of the reasons for its poor fuel economy is the reason for its almost electric smoothness and hp/liter advantage is that each rotor fires three times per revolution, meanwhile the 4 stroke cylinder engine one cylinder only fires once per 2 revolutions. this means ur 4 banger fires 4 times in 2 revolutions, meanwhile the rotary fired 12. In order to get that kind of number and smoothness you'd have to look into a 12 cylinder engine and even then the eccentric design of the rotary allows for a smooth application.
also the design with no valves makes the rotary's scavenging situation undesirable which causes fuel efficiency problems, from what i understand the new Renesis motor works on some of these issues. And efficiency as a whole of the "system" decreases, because the oval shaped housing increases the surface area for heat loss to the coolant. there are other losses to account for too, i'm just tired of typing.