Unable to change the size of the bulk thread pool

I am running ElasticSearch 2.3 and indexing large numbers of documents using the Bulk API. I wanted to tweak the size of the bulk thread pool to see how that effects performance, but it appears ES locks the max to be 1 thread per core, even though the docs do not mention this as being the case.

Am I able to increase the size of the Bulk thread pool?

Thanks,
Craig