This is from a pitching site: An overweighted ball, when used correctly, can be an excellent aide in developing strength and pitching speed. We do NOT recommend the 1 pound ball you may have seen advertised because we feel its excess weight poses a risk of injury for some girls. Similarly, balls that are only slightly overweighted (7 oz. or 8 oz.) have had minimal effect for many pitchers. Since the regulation softball is approximately 6? ounces, we encourage our pitchers to use a regulation size (12" circumference) "training" ball that is no more than 9 oz. to 12 oz. (depending on age, strength, and physical development). However, even at this weight, it should NOT be thrown at full speed. Follow the instructions that come with the ball, and be careful!
If you have attended a Bill Hillhouse clinic he is not in favor of them. He may have comments on his web page on this subject.