As an additional concrete detail to Edward's response, 'result pinning' can provide some performance improvements depending on topology and workload. See the conf file comments for details: https://github.com/apache/cassandra/blob/cassandra-0.8.0/conf/cassandra.yaml#L308-315
I would also advise to take the time to experiment with consistency levels (particularly in multi-DC setup) and their effect on response times and weigh those against your consistency requirements. For the record, any performance twiddling will only provide useful results when comparable metrics are available for the similar workload (Les, it appears you have a good grasp of this already - just wanted to re-iterate).
