[Barnsnake]

Interesting question, but not a simple one.
In the example, we were at the limits of adhesion and assuming somewhat neutral handling. Using the same parameters, with additional rearward weight bias from your "ballast" you would rotate off the road even faster due to a greater pendulum effect.
However:
Adding ballast to the rear would change the overall handling characteristics of the car. If you added enough grip to the rear (relative to the front) it might just try to understeer straight off the road long before you neared the limits of adhesion of the rear. Lifting off the throttle or braking in an attempt to arrest the understeer might result in snap-oversteer and again, backing off the road.

Weight (MASS, actually) is ALWAYS the enemy when trying to accelerate, stop, or turn. Increasing rear grip by increasing the coefficient of friction (big, sticky tires) is better than by increasing the force normal to the plane (ballast).