Whereas high-power operation (> 1 W of cw output power at 200 K) has been demonstrated for quantum cascade lasers emitting at λ = 4.7-6.2 μm, those devices generally exhibited multiple longitudinal modes. Recently, a distributed-feedback quantum cascade laser operating in a single spectral mode at λ = 4.8 μm and at temperatures up to 333 K has been reported. In the present work, we provide detailed measurements and modeling of its performance characteristics. The sidemode suppression ratio exceeds 25 dB, and the emission remains robustly single-mode at all currents and temperatures tested. Cw output powers of 99 mW at 298 K and 357 mW at 200 K are obtained at currents well below the thermal rollover point. The slope efficiency and subthreshold amplified spontaneous emission spectra are shown to be consistent with a coupling coefficient of no more than κL ≈ 4-5, which is substantially lower than the estimate of 9 based on the nominal grating fabrication parameters.