See also a separate related answer by Patrick Gupta, second answer to Did merging Black Holes in GW150914 give up entropy and information to the gravitational waves, since they lost 3 solar masses? (the question was faulty). He calculated the final horizon area for the rotating case, with entropy 1.57 times the original entropy, so entropy did grow, and the second law of BH thermodynamics held up. Using the Kerr solution and equations for the horizon area, as Peter Shor did, is important to do because unless it is a heads on collision (very unlikely) and there was no individual rotations to begin with, the final black hole is very likely to have a significant angular momentum. It is interesting, and should not be surprising, that the observed merger led to the high angular momentum (a=.67) observed in the final black hole.
Worth noting that Hawking derived first the limits for both rotating (edited, as indicated correctly by Michael Seifert in a comment below) and non rotating bodies in 1971 Phys Rev Let, and published more in 1972 for both rotating, non-rotating, and charged bodies (though he may not have been the first on the latter) in the Les Houches Summer School Lectures on Black Hole in 1972. For rotating Kerr black holes the max is just 50%, and for rotating and charged Kerr Newman black holes it's approx. 65% max.
The many numerical and PN calculations done (by Smarr and others), as stated by Lawrence Crowell in his nice answer, over the years and before the LIGO finding eventually led to an understanding that 5% or so was a more likely number in many cases (not sure if those include charged, those are not likely to occur astrophysically).
It's worth noting also that for LIGO they could not get a measurement/estimate of the initial BH rotations, if any, and estimated it would have made only a small difference (smaller than the mass uncertainties estimates) on the final estimate for the radiated grav energy. In later observations they expect to see more earlier and maybe get any pre merger rotation rates.