Drilling a slug has nothing to do with the warranty. Unless you drill any of the holes inside of 1 3/16" to the pin or MB(in the case of a drilled in PSA marking) then you should get a replacement. Where were you storing the ball?
It is clearly stated on the box of Storm balls, and probably all other companies for that matter, that slugs and insert usage void warranties. Now if the company decides to honor the warranty anyway, that is a different issue.
I've drilled a quite a few IQ pearls, with no problem. Often, if the pilot hole for the slug was not beveled properly, or not opened up enough for the slug to fit properly, this can cause the issue of cracking. Pounding and jamming slugs into a hole that is too snug can create weakness to the outside cover of the ball.