Voting bots will likely be strongly deterred by both the addition of a certain amount of free downvotes from a separate pool, as well as more competitive rewards elsewhere in the form of higher curation.
It's difficult to say what the ideal level of voting concentration is other than describing it loosely as the level of concentration that would actually reflect the distribution of rewards if stakeholders voted honestly based on their opinion of the content, and that's precisely what we're going for.