Since surface area of the remnant black hole must be more that the sum of the binary surface then the maximum energy released via gravitational waves is
$$
ΔE = [M_1 + M_2 – \sqrt{M_1^2 + M_2^2}]c^2
$$
That means that when ratio of the initial masses is very large then almost all mass of smaller black hole may turn into gravity waves.
I went through "SXS Gravitational Waveform Database" – it has 8 simulations of mergers with ratio above 9. For all of them less then 1% of total mass is converted into gravitational waves while typical LIGO merge releases several percents. This implies that in typical super massive BH and stellar mass BH merge there is almost no emission.
How does typical (most likely in the wild) and extreme (with maximum loss) case of high mass ratio merge looks like?
Best Answer
If I understand "Gravitational wave snapshots of generic extreme mass ratio inspirals" (thanks to @andrew for the tip) correctly then typically almost all smaller mass body is converted in waves.
, where μ is the mass of the smaller body and E is wave energy.
But for head-on collision (plunging orbit) conversion ratio will be very small (see "Gravitational radiation from plunging orbits").