: This paper analyzes the performance characteristics of a high-speed aerospace surface-mounted permanent magnet synchronous motor (SPMSM) with different pole–slot combinations under identical rated power and stator outer diameter conditions when Fe–Co electrical steel (Hiperco 50) is applied. The target motor operates under two operating points: a high-speed, low-torque condition and a low-speed, high-torque condition, and the magnetic saturation limit is determined based on the low-speed operating point. First, reference models using conventional Fe–Si electrical steel (35PN230) are designed for three pole–slot combinations and compared under identical current density and winding conditions. Then, the additional saturation margin provided by the higher saturation flux density of Fe–Co electrical steel is utilized to implement three design approaches. The first approach reduces the stator tooth and back-yoke dimensions to achieve weight reduction, the second increases the number of turns to expand electric loading, and the third increases the magnet volume by enlarging the rotor outer diameter. Finite-element analysis is conducted to compare the loss characteristics and power density at both operating points. The results show that the performance variation resulting from Fe–Co electrical steel strongly depends on how the additional saturation margin is utilized, and the loss and power-density characteristics vary significantly with the pole–slot combination. This study investigates the influence of pole–slot selection and high-saturation magnetic materials on the design of high-speed aerospace SPMSM under an identical stator outer diameter condition.