The main issue here could be that a motor-generator system is capable of handling a much higher electric INPUT than it can OUTPUT as long as the motor is powering something that is producing resistance to the rotation when it is receiving energy and this resistance is greater than the maximum electromagnetic the resistance the motor can produce as a generator. This is because the RPM on the output (power input) is limited by the torque the engine is applying to the system, i.e. propelling the car up a hill, while when the motor is generating power, the only torque available to generate power against is created by the electromagnetic fields of the magnets, presumably a much smaller than the weight of the car etc.
If you applied the max power that the motor can handle without a torque to resist the rotation, the motors would exceed their maximum rotation speed and fail. Not accounting for lots of variables, this is the speed you would need to achieve to generate the same power. To generate the maximum power that the batteries could absorb while warm, say 120kw, the motors would have to be spinning much faster than they currently are. The gear ratios are optimized for acceleration and energy efficiency, and since they are fixed, they cannot be adjusted to increase the generation RPM of the motors and thus generate more energy when regen braking.
I have never tried regen braking from 150mph, in theory you would be able to achieve much higher power, but I assume that Tesla has placed limitations on actual regen power to keep the system stable or within safe bounds for the components involved, which are likely separate from supercharger components that can handle higher currents. Clearly they have an algorithm that limits regen at high speeds for reasons unknown to me, but I suspect that at normal highway speeds, regen is maximized.
This is why the max output power is 250+kw, but the max regen is about 50-70kw. It's either a limitation of the components that handle regen current, or a limit on the actual regenerative capacity of the motors at RPMs highway speeds can provide, or a combination of both. My experience is that I can achieve max regen probably around 40mph, indicating that regen at speeds higher than this are limited by the battery and current limitations on regen components. It's also possible Tesla wants to closely control the regen experience, they don't want it to be too strong. That could pose safety or ride quality issues.
Who knows, lots of possibilities!
