There are studies that support that throwing overweight and underweight balls improves velocity. Here is an overview of the studies: Effects of throwing overweight balls
As far as injuries, the ASA didn't establish a blue ribbon panel of experts in physiology, anatomy and sports science to determine the optimum weight of a softball. Saying that throwing a ball one or two ounces heavier or lighter is inherently more dangerous doesn't make much sense.
Every softball parent keeps a hockey mask and a butcher knife in their car...
What program of Kempf works? That chapter included?
Bill Hillhouse had some good comments on weighted balls. I consider him a good source. He posts on here all the time.