I am surprised I never see mention of http://synergyaircraft.com/ when aircraft efficiency comes up here. The designer/builder knows his stuff, and the models and tests have proven the double box tail is incredibly efficient.
I really hope they manage to finish the prototype and get it flying!
One of the claims made here is "Synergy uses brilliant physics to achieve twice the expected speed for a given horsepower."
The power required to fly at a given speed is the product of that speed and the drag at that speed. If this is truly an apples-to-apples comparison, it implies Synergy is producing half the drag of the reference airplane at twice the speed. Drag of an airplane goes approximately (a good approximation, when we are talking about maximum achievable speeds) with the square of the airspeed, and therefore this is implicitly a claim that the these innovations reduce the drag coefficient of Synergy by (by, not to) 7/8.
Given the effort it takes to reduce sailplane drag by a few percent, and the enthusiasm within that community to adopt whatever it will take to do so, I will be skeptical until I see a quantitative explanation based on actual measurements.
It is claimed that the innovations mainly reduce the induced drag, but this makes the claim even more surprising, as induced drag decreases with speed, and so, therefore, do the benefits of reducing it.
That certainly looks like it pulls more aerodynamic tricks than this mysterious Celera, which is fairly conventional-looking besides the pusher prop and the non-cylindrical fuselage.
I really hope they manage to finish the prototype and get it flying!