The bumps don't enter the equation. I've observed that behaviour long before the bumps were added.
The problem with balancing the AI is the sheer number of variables. There's several kinds of cars (I mostly used the URD GT cars during testing) and there's several AI settings that will influence their behaviour and speed (AI skill obviously, AI aggressiveness, some specific AIW settings and of course the beef of the AIW: the AI line/corridors).
There's a setting in the AIW that allows for balancing top speed, acceleration, braking and cornering against each other. Problem is that these are linked to the AI strength you set. So if Cornering was set to 50% (with the remaining 50 spread between the other three) that would make the AI corner better at say 110% AI strength. At higher strengths cornering gains more. At lower strengths though cornering loses more (as it is set to the highest weight of the four).
For Le Mans the ratio is 22.5 each for acceleration and braking, 25 for top speed and 30 for cornering.