I'm not an engineer, but in one of my past lives I raced on track and in rally. Whenever we needed to add weight, we'd keep it as low as possible, as central as possible, and always between the wheels. Adding weight, in itself, is detrimental to handling, acceleration, deceleration, normal wear, and increases failures of various components. Placing added weight outside the wheels compounds issues with braking stability and handling, as it adds to the pendulum effect. One might not notice much difference in normal everyday driving, but would certainly notice the difference when approaching the limits of the vehicle, such as taking evasive maneuvers to prevent a crash or if they need to come a quick stop on a slippery curve. Everything else being equal, adding weight past the rear tires would lead to oversteering at lower speeds and increased chance of loss of control.