This study presents experimental characterization of the HyPEx-B high-density hybrid rocket fuel comprising aluminium, bismuth trioxide and HTPB binder with nitrous oxide across a range of oxidizer to fuel ratios near the theoretical performance optimum. The oxidizer to fuel ratio was varied by changing grain length while all other motor hardware remained identical to the previous study. Fuel regression was quantified by comparing grain mass on a calibrated scale and by volumetric analysis of longitudinal cross-sections, with both methods showing good agreement. Peak delivered specific impulse of 200.8 s was measured at O/F=2.98 with a combustion efficiency of 97% compared to AFAL and peak volumetric specific impulse of 1818 Ns/dm3 at O/F=2.62. While mass-specific impulse is comparable to reference HTPB based formulations, the volumetric specific impulse shows a consistent advantage attributable to the high-density bismuth trioxide loading. The paper also briefly explores potential paths for the future development of this type of compositions.