We study spatial intensity distributions in plasmonic distributed feedback lasers (DFB) composed of metal nanoparticle arrays. Real-space distributions give direct access to “coupling-strength” parameters that quantify DFB performance in the framework of coupled-wave theory (CWT). We observe that CWT indeed parametrizes real-space intensity distributions and extract coupling-strength parameters that quantify the plasmonic feedback mechanism. These coupling-strength parameters differ from those required to parametrize the plasmonic band structures of the system, counter to the common result for dielectric DFB lasers, where CWT describes both real-space and k-space physics. Also, the measured coupling constants are significantly smaller than would be expected from estimates on the basis of the unit-cell geometry. We conclude that while CWT is successful as a generic description of any system with forward and backward waves with gain, matching this model to photonic band structures, or to common parameter-estimate approaches, fails because the underlying assumption that a perturbative plane-wave expansion applies is not valid for plasmonic antenna arrays.