Is the commonly assumed UFO (flying saucer) design aerodynamically efficient?
They use the concept of VTOL thats vertical Take off and Landing. Click here to know more about VTOL

Regarding the efficiency, well I think no one can give a 100% perfect information on this because, we do not know the fuel used, the engineering behind the structure. There are a lot of factors affecting efficiency like :
Materials, Fuel, Type of Engine, Weight etc. So aerodynamics alone cannot judge the efficiency of the aerovehicle. 
Thanks!