It's also not clear that a better filter will result in a substantial improvement for SQLite. If the improvement is on the order of a tens or hundreds of nanoseconds per a query, that's nothing compared to hitting disk.

Edit: I guess you could get really efficient at detecting things not in the database, but the soon as you get a single false or true positive the process runs into the equivalent of a screeching halt if you need to actually check if it's there.