How to scale logstash-kafka

We are using the logstash-kafka input with this configuration:

kafka {
    #Kafka Consumer Group ("logstash" is default, specifying for clarity)
    group_id => "logstash"
    white_list => "topic1,topic2,topic3,...,topicN"
    zk_connect => "connection-info"
    codec => "line"

I know that it is important to maintain a balance between consumer threads and partitions. How does that affect our ability to scale via adding logstash instances?? For example if we are consuming from one topic with a partition of 1, we can't actually scale up simply by adding a logstash instance, because more consumer threads than partitions = idle threads.

Likewise if we start to add more topics to our whitelist, we can end up with more partitions than consumers. Can logstash-kafka be configured such that it scales easily without losing consumer-partition balance??

Other than having multiple partitions I think Logstash 1.5.3+ has more parallelism in its pipeline so as long as the Kafka threads are keeping up you'll still get performance. if your topics are lagging you can always split them between Logstash instances. Not exactly horizontal scaling but it'll help. Otherwise if you want to horizontally scale topic consumption you need to have partitions.