I don't think the downforce converts to more traction linearly. Maybe it would if you could size the tire as downforce is increased or decreased. In other words there's a point where traction increase isn't as much as the amount of weight added.
However, inertial weight does convert to inertia linearly. Double the weight and you double the inertia.
Why do big heavy vehicles take longer to stop even if they have enough braking power to lock the wheels? Could it be from very little tire in comparison to weight? Compare that to a sports car with wide tires. Obviously width isn't everything, but does seem to play a role on traction. Adding more weight would seem to merit bigger/wider tires in proportion to the weight increase.
Adding more downforce doesn't increase traction linearly, but doesn't add inertial weight.
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