Note though that even if we don’t give suffering greater weight than pleasure (and thereby switch to average utilitarianism), the point to stop population growth must be in exactly the same place because that’s where the average component of change that can be classed as a gain starts to be outgunned by the average component of change that’s classed as a loss.
That’s not how the math works out, actually. If you have found a point where the Benefit and Suffering curves (as determined by average utilitarianism) are such that the derivative of the sum of the curves is zero (or in other words, the derivative of one curve is equal to negative the derivative of the other curve), then multiplying one of the curves by some quantity not equal to one will make the derivatives no longer equal (assuming that the derivatives are not equal by virtue of being zero). This is because the derivative of a function times a constant is equal to the same product times the derivative of the function.
That’s not how the math works out, actually. If you have found a point where the Benefit and Suffering curves (as determined by average utilitarianism) are such that the derivative of the sum of the curves is zero (or in other words, the derivative of one curve is equal to negative the derivative of the other curve), then multiplying one of the curves by some quantity not equal to one will make the derivatives no longer equal (assuming that the derivatives are not equal by virtue of being zero). This is because the derivative of a function times a constant is equal to the same product times the derivative of the function.