Soul of Stone won't affect it. You gain the corruption and insanity before you actually spend experience and gain any talents.
And it just occurred to me that that includes spending experience on willpower advances, so I didn't have as much willpower as I thought.
Edit: In fact directly before you roll for the insanity and corruption you'll have just chosen your homeworld and career, and it specifically says that no experience can be spent on backgrounds or advancements yet. So yeah you get the willpower that you get from your rolls and homeworld and that's it.
Which makes sense really. It's trying to simulate a lifetime of fighting chaos. It would be expected to have racked up some trauma in that time, and if you get the advantage of taking all your experience points first you really wouldn't be in much danger.
Still there isn't much chance of rolling 30 corruption. It's only d5, and even without advancements most people have at least 20 or 30 willpower. Enough to reduce it to 6d5 or less.