The efficiency of both processes reaches optimum when both bodies are of equal mass: two Black Holes accelerating towards each other and coalescing. The intense gravitational field around a Black Hole perturbs the space and time around it. As the Black Holes move through space, they leave behind a large wake of bent space-time waves, which travel away at light-speed. These are gravitational waves and for un-charged Black Holes can radiate away as much as 10% of the mass of each Black Hole. The mass has indeed somehow managed to escape from within the Schwarzschild radius, but this is no surprise, both are the same thing - gravity.
Fully, but oppositely, charged black holes can radiate away as much as 43% of their mass. Oppositely charged and oppositely spinning Black Holes can radiate away 90%?? of their mass. These processes can never be 100% efficient; a Black Hole or space-time singularity must always be present afterwards. See 'Nucleosynthesis by Neutron Star Mergers'
Except for matter/anti-matter annihilation, which is 100% efficient, these are by far and away the most efficient ways known of transforming energy from one form into another, and happen because gravitational energy is negative: when the Black Holes were apart they really wanted to be together; who separated them and with what energy? That is indeed another question!
See White Black Holes