This is something that as far as I know is not covered by any FAQ, Rule Book, or Army Book provision.
It is worth noting that the same issue arises with Mnsters and Handlers when the Monster dies, and there is no FAQ or rule directly covering that situation either.
Some people would argue that the situation should be analogized to killing blow against a multi-wound model. In that case, you count the full remaining wounds of the model towards combat resolution.
However, in the KB case, you count all the wounds that would have otherwise been necessary to remove the model. In the case of the Steg and Monsters and Handlers, killing the crew is not necessary to remove the model. As a result, I don't think remaining crew should be counted for the purposes of combat resolution.
That's just my interpretation though. Again, it's not really covered by the rules as presently written.