I think you are barking up the wrong tree.
I think you are far more likely to benefit from mechanical tweaks and from increasing the strength and flexibility of your core than you to benefit from a weighted ball program. This is because the key source of a pitcher's power (e.g. 80%) is the properly sequenced rotation of the hips, torso, and shoulders.
No weighted ball program will help you work on that.
If you want to consider this approach, then the idea is to get your hips rotating well before your shoulders. That stretches the muscles of the torso which enables them to powerfully pull the shoulders around.