Purpose: To evaluate how primary endpoint selection influences patient eligibility, enrolment, and detection of treatment efficacy in RPGR gene therapy trials, comparing low luminance visual acuity (LLVA) with microperimetry-based outcome measures. Design: Retrospective analysis of a phase 1/2 to 3 interventional clinical trial. Subjects: Fifteen patients were included in this study. Each was administered with gene therapy in one with the other eye serving as control. Methods: Month 12 data from patients with RPGR-associated retinopathy enrolled at a single center in the XIRIUS trial (NCT03116113) were retrospectively analyzed. FDA-aligned low-luminance visual acuity (LLVA) responder criteria (≥15-letter gain) and EMA-aligned significant change from baseline in microperimetry mean sensitivity, were applied. Microperimetry outcomes were evaluated across the full 68-point MAIA grid and within a central 16-point subset. Minimal baseline inclusion thresholds were used. The proportion of gene therapy–treated patients meeting FDA- and EMA-aligned (≥2.5 dB improvement in mean sensitivity) responder criteria was determined. Main Outcome Measures: Proportion of patients meeting the responder criteria in either microperimetry or LLVA. Results: At baseline, LLVA excluded more patients from study inclusion due to floor effects than microperimetry. At month 12, 2 of 10 patients (20%) met LLVA responder criteria. In contrast, 5 of 11 patients (45%) met EMA-aligned whole-grid microperimetry responder criteria, including all LLVA responders and 3 additional patients with clear functional improvement not captured by LLVA. Restricting analysis to the central 16 microperimetry points further increased responder detection, identifying 7 of 11 patients (64%). Improvements in microperimetry mean sensitivity of at least 2.5 dB, exceeding expected test–retest variability. No responders were identified for any endpoint in untreated fellow eyes. Conclusions: Primary endpoint choice substantially affects efficacy detection and patient inclusion in RPGR gene therapy trials. LLVA demonstrated limited sensitivity and restricted eligibility to a narrow disease window, risking underestimation of treatment benefit. Microperimetry mean sensitivity, particularly when spatially aligned with the treated retinal area, detected functional improvement in a substantially larger proportion of patients and supports broader enrolment. Microperimetry-based endpoints provide a more sensitive and inclusive primary outcome measure for RPGR gene therapy trials.