Just wanted to clarify that's no longer the case, and they did put it in the FAQ.
In the heady pre-battletome days of GA: Chaos, you could go over, and I killed many a Bloodthirster/unit with the shenanigans. But now you can do a Max of 10 if you don't heal him.
Q: I would like some clarification on how the Jabberslythe’s Spurting Bile Blood ability is intended to work, specifically whether ‘overkill’ damage triggers dice rolls to test for mortal wounds. For example, if my Jabberslythe suffers 12 wounds from a single unit’s melee weapon(s), do I roll 12 dice to test for mortal wounds even though the Jabberslythe has a Wounds characteristic of 10?
A: The Spurting Bile Blood ability triggers when a wound is allocated. As wounds are allocated one at a time until the model is slain, any ‘overkill’ will not cause the ability to be triggered. See page 7 of the core rules, ‘Allocating Wounds’.