Can Two Ultrarelativistic Objects Lose Almost All Their Energy to Gravitational Radiation?

In 2007 Pretorius and Khurana did "speculate that at threshold [at a critical impact parameter], all of the kinetic energy of the system [two ultrarelativistic black holes] is converted to gravitational waves, which can be an arbitrarily large fraction of the total energy." However, in 2012 Sperhake, Berti, Cardoso, and Pretorius performed numerical calculations that led them to the contrary conclusion: "An extrapolation of our results to the limit $\gamma \rightarrow \infty$ suggests that about half of the center-of-mass energy of the system can be emitted in gravitational radiation, while the rest must be converted into rest-mass and spin energy." Here I present arguments against this latter conclusion and in support of the earlier speculation that for sufficiently large $\gamma$, all but an arbitrarily small fraction of the total energy can be radiated away.