> An analogy of filling a curved-bottom tub would be a better fit
No, it would not. That rationalizes your concept of trying to match the input distribution with the response function, but it is specious and incorrect to apply it to a response function. Filling a tub with liquid allows liquid in one place to move to another place. Light doesn't work that way, light at 550nm doesn't spill over to 600nm. The incoming light at 550nm is completely accounted for in the output response, and doesn't affect the response at 500nm or 600nm, and isn't affected by input at 500nm or 600nm.
The response function is literally an efficiency curve. If you want maximum efficiency or maximum output, you give it inputs that land at the apex of the efficiency curve.
> I would also suspect that the receptors are becoming less and less sensitive to the stimuli they accept as they get closer to saturation
That is correct, but outside the bounds of what this article is discussing. For all practical normal daylight situations, which the range of all monitors lands inside, response is roughly logarithmic (or linear on a log scale). Clamping or saturation in the response happens in extreme darkness and extreme brightness. Response to the night sky in between stars, or to staring directly at the sun have a non-log. Response to computer monitors and almost everything you see during the day is linear on a log scale.
> If that really is the case, then a LF really would be better off if it was more bell-shaped
It's extremely unlikely that there's any real-world scenario under which this is true. But if you want to do this thought experiment, then you need to account for the receptors saturating. If you push the receptors to saturation, then their entire response goes flat, changing the overall shape of the response function. By matching the shape of the response function and then saturating, you force your target to move, and then get the wrong answer anyway.
If you push one receptor to saturation, the next best answer would be to put the remaining portion of your power distribution at the apex of response sensitivity for the other two receptors, so you'd have three specific wavelengths, but not a bell shape.
Here's an deeper discussion of human visual response that includes cases of extreme ranges and saturation: http://www.telescope-optics.net/eye_intensity_response.htm
Look specifically at figure 242. Monitor level brightness response is very near the center-line (vertically) in the chart. All normal daylight conditions are represented between the "cone threshold" (0.001mL) and "discomfort" (100,000mL) marks vertically. You will note that no response clamping occurs between these values.