Can binary mergers produce maximally spinning black holes?

Gravitational waves carry away both energy and angular momentum as binary black holes inspiral and merge. The relative efficiency with which they are radiated determines whether the final black hole of mass Mf and spin Sf saturates the Kerr limit (χf≡Sf/Mf²≤1). Extrapolating from the test-particle limit, we propose expressions for Sf and Mf for mergers with initial spins aligned or anti-aligned with the orbital angular momentum. We predict the the final spin at plunge for equal-mass nonspinning binaries to better than 1%, and that equal-mass maximally spinning aligned mergers lead to nearly maximally spinning final black holes (χf0.9988). We also find black holes can always be spun up by aligned mergers provided the mass ratio is small enough.