  We explore the impact of black hole spins on the dynamics of high-energy
black hole collisions. We report results from numerical simulations with
gamma-factors up to 2.49 and dimensionless spin parameter +0.85, +0.6, 0, -0.6,
-0.85. We find that the scattering threshold becomes independent of spin at
large center-of-mass energies, confirming previous conjectures that structure
does not matter in ultrarelativistic collisions. It has further been argued
that in this limit all of the kinetic energy of the system may be radiated by
fine tuning the impact parameter to threshold. On the contrary, we find that
only about 60% of the kinetic energy is radiated for gamma=2.49. By monitoring
apparent horizons before and after scattering events we show that the "missing
energy" is absorbed by the individual black holes in the encounter, and
moreover the individual black-hole spins change significantly. We support this
conclusion with perturbative calculations. An extrapolation of our results to
the limit gamma->infinity 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.
