I only have time for a relatively short answer...
If the car already has a soft suspension, and the softest tires allowed for per your race rules, and you still don't have enough grip (four wheel drifting),
then adding weight will help.
However, if the car is not four wheel drifting, then you really should just
change the front to rear
weight distribution (or the suspension). When using weight distribution, you would want to shift the weight of your current compenents (battery, motor, electronics) away from the end that has enough grip and toward the end that is lacking in grip. If you just add more weight to the end of the car where grip is needed, then the cars
potential cornering speed will decrease
from what it could be, if the current front to rear grip were balanced correctly (by weight distribution or suspension setup).
Keep in mind that when shifting weight, it is always best to:
1. Move it closer to the axle that needs more grip,
but keep it in between the front & rear axles. Never in front of the front axle, or behind the rear axle. If you do, when the grip of the track comes up, the car will tend to have off-power oversteer (too much weight in front of front axle) or on-power understeer (too much weight behind the rear axle). This is due to a pendulum effect where the car "see-saw's" over the affected axle from the improperly distributed weight.
2. For all heavy items, keep the weight as close to the centerline (belt) as possible, while moving it closer to the axle that needs more grip. This makes the car transition from left to right quicker and with more stability, while adding the grip needed.
3. Keep the weight as low as possible. This helps increase cornering speed as well as make the car transition quicker.
Believe me, there is a lot more to understand.