The I-V characteristics, lasing thresholds, and wallplug efficiencies of type-II "W" mid-IR diode lasers from 16 different wafers were studied in order to determine the influence of various device parameters. At T = 90 K, the wallplug efficiency for a 1-mm-long gain-guided device was > 10% and the slope efficiency was 142 mW/A (38% external quantum efficiency). When a 22-μm-wide ridge was lithographically defined on a 5-period "W" laser with a p-GaSb etch stop layer, the maximum cw operating temperature increased to 230 K. We also investigated 5-stage and 10-stage interband cascade lasers containing "W" active quantum wells. For 10-stage devices, the low-temperature threshold current densities were somewhat higher than in the "W" diodes while at higher temperatures they were slightly lower. The threshold voltage was only ≈ 0.1 V larger than the photon energy multiplied by the number of stages, corresponding to a voltage efficiency of > 96%, while the differential series resistance-area product above threshold was as low as 0.21 mΩ.cm2 at 100 K. At T = 78 K, the cw slope efficiency was 0.48 mW/A (126% external quantum efficiency), and a maximum cw power of 514 mW was produced by an epi-side-up-mounted 2-mm-long 10-stage laser cavity with uncoated facets. A 5-stage 2-mm-long interband cascade laser produced ≈ 700 mW of output power at 80 K, with a maximum wallplug efficiency of 20% per facet.