Lifetimes, Auger coefficients, and internal losses were deduced for 25 different type-II "W" interband cascade laser structures, from correlations of the experimental threshold current densities and slope efficiencies with calculated threshold carrier densities and optical gains. The room-temperature Auger coefficients for a number of lowthreshold devices emitting at wavelengths from 2.9 μm to 5.2 μm fall in the narrow range 3-11 × 10-28 cm6/s, which represents a much stronger suppression of Auger decay than was implied by most earlier experiments and theoretical projections. The estimated internal loss is lowest at intermediate wavelengths, and the most recent designs display additional reduction to as little as 8 cm-1 at 300 K.