That is not correct. I just conducted an experiment to verify.
Coasting down my hill in the Model S, regen was ~40kw when I applied the brakes because I was 8mph over the speed limit and needed to slow down. Regen immediately dropped below 30 and then went to zero of course when stopped. Max regen is 60 kw - I wasn't there.
Same scenario in the Karma. Coast regen was ~30kw, applied the brakes at the same speed as above, regen jumped up to 70kw and then dropped off to zero when stopped.
I *do* understand what you've said, but I had a thought on it. (I will also try this on my own but it would be nice to have corroborating observations.)
It is a known fact that the Model S will be regen'ing at a certain rate (let's say 40 kW for sake of consistency), and as the vehicle's speed decreases, the regen rate will diminish gradually, until it is near 0 kW as the car approaches 0 MPH.
Your observation is that in your context, the Model S was regening at 40 kW until you applied the brakes. You hypothesize that the car has decided to reduce the regen rate because of the application of the brakes. Do you think it's possible that by applying the brakes, you slowed the car's speed down to the point that lesser regen was being achieved due to speed alone (and not a car's decision)?
It would clarify your hypothesis that you were to take a video of the dash display on your same hill, performing two separate test runs (you start with the vehicle at the same speed for both runs). The video must show your regen rate alongside your speed, for comparison. Your runs differ thusly:
a) remove your feet from both pedals so that regen rate is seen without braking.
b) remove your foot from the go pedal, and apply the brake (perhaps gently).
You can then say for sure that you achieve X kW of regen in test a at a given speed (e.g. 20 MPH), but Y kW of regen in test b at the same speed.
It may be useful to have a test c) where you apply the brakes with greater force.
The reason I suggest video is because it is difficult to look at the regen dial while at a certain speed, if your speed is changing (it takes your eyes too long to move and read the number).
Does that make sense?
Also, at one point you said:
It wouldn't be safe to generate .25G of deceleration through regen just by lifting off the accelerator, as it would be too abrupt and your brake lights wouldn't be on to warn the traffic behind you that you were slowing down that much.
But in the Model S, your brake lights do illuminate without you touching the brake pedal, if the deceleration rate is high enough. You can "feather" the go pedal to keep the "needle in the green between 3:00 and 4:00" and avoid brake light illumination, but above a certain deceleration, the brake lights will illuminate. I do not know (maybe I should get that iphone app) what G-rate will trigger the brake lights.