Because the motor is IPM not SPM, it takes an additional look up value for rotor reluctance torque compensation to field weaken efficiently. The beautiful thing is, SPM motors rarely can field weaken more than 70-100%, and some IPM motors can field weaken up to 250% of base speed or more. Nissan leaf is an example of this. The more active iron mass to magnet mass in the rotor, the more the motor motor will be capable of efficiently exceeding it's base RPM*Kv speed. Motors that support lots of field weakening are magnetically sitting closer to a "stepper" motor when making peak torque at low RPM's, but they are exclusively "stepper" motor type drives when field weakening way above base speed, the PM function is supplementary and to enable efficient high torque density and control-ability.
It's not uncommon to eat 2-6% motor efficiency to extend useful RPM powerband by +100% in a high iron% IPM rotor motor, where an SPM motor might be eating 10% efficiency to get +50-60% over base speed.
At some point rotor magnet strength being high is almost a penalty to motor performance, and you end up just wanting the highest temperature permanent magnet material, and then using minimal amounts of it in a mostly iron rotor to leverage reluctance torque.