In short, it was bad. There was a huge air bubble, which made for a large gap visible in the middle of the grip. In addition, a lot of the plastic was absorbed into the ball. Weighing this ball and a fresh one revealed that 50 grams of plastic was absorbed (most likely through the seams and holes where the seams come in and out of). Finally, the plastic that did stay on the ball felt squishy and weird, probably indicating that my ratio of amounts was not quite correct. However, it was not a complete failure. The plastic stuck well to the leather of the ball, meaning that the overmolding concept had potential.