I'm referring to bucket IO priority within Couchbase Server. This has nothing to do with client->server TCP connections or IO service.
Couchbase 3.0 changed the threading model for buckets in Couchbase Server. Rather than assigning a specific number of threads per bucket, it moved to a global thread pool where certain buckets could be assigned either low or high priority (meaning they get a differing percentage of access to disk IO). The API's threadNumber parameter was kept, but the meaning was changed. 3 threads = low priority. 8 threads = high priority.
The SDK currently sets 2 threads, which does not conform to the 3/8 convention. Since the default bucket priority for manually created buckets is low, the SDK should also use this default for programmatically created buckets. That requires threadNumber=3.