With regenerative braking, what speed does additional KW generated cease?

Well, what you wrote is true... it's just that it's only the tip of the iceberg. The other 85% happens when you're on the wide pedal.

 

Electric power generation is not just a matter of RPM on the generator. It also involves torque on the generator shaft and electric load applied to the output terminals.

A generator can spin very fast, but provide no power. If no power is drawn from it, then it will not produce any torque resisting whatever is driving it (except small bearing and windage frictions).

Or it can be spun slower, but have a lot of power drawn from it, it which case its drive shaft will produce a lot of torque to resist whatever is driving it.

The car's computers can control and adjust the amount of load put on the generator, based on requests from your brake foot or from other car propulsion or battery charging or accessory items.

 

fuzzy 1 said that well. And while you are thinking that way, you see why the Prius resists having the battery 'full'; it reduces the choices the computers have to store electricity while braking.

Because the car's kinetic energy goes up as the square of the velocity, at high speed the motor is limited in how much of that energy it can regenerate. (and at very low velocities, there is too little energy to harvest)

So at freeway speeds, if you are forced to brake, much is wasted as heat, and below 7 MPH there is nothing to store. Planning ahead so you don't brake at all is always the best tactic.