I can see with ACIM motors that using FW is never a problem. I guess even 100% FW is possible.
PMSM though i can see even more than 25% FW can be a problem. I guess because of magnet excitation and winding current blanketing torque drops off faster.
With Emrax motor for example because of many pole count FW stability is difficult at more than 5000rpm.
Maybe this was the reason Toyota went for boost system rather than FW up to drive speed. Maybe they made analysis that yes 30kW is enough for normal speed and if you use FW you may get even more losses at the same torque/speed.
I am rather interested in Lexus drive and its FW capability. This would be a worthy test bed to try efficiency of both ways to get to high RPM.