How can an all flying rudder be almost 100% more efficient than a vertical tail and rudder?
According to the Zenith school of aircraft design, a rule of thumb for the size of an all flying rudder is 7% of wing area. For a classic vertical tail and rudder, the rule is 12%. That's an increase in area of almost 100%!!
A classic rudder foil, something like a NACA0012, has a max cL of roughly 1.5.
A vertical stabilator and rudder is like a wing and an unslotted flap, which adds cL of about 0.7. That's a total cL of about 2.2, which works out to an increase in efficiency of 50%.
So... why is an all flying rudder 100% more efficient, if a vertical stab and rudder produce 50% more lift?