How is an export/import operation different when it happens within the same instance as opposed to when it happens between two different instances. Is there a performance difference between a two?
I suspect there may be confusion about what an instance is. A Streams instance is responsible for managing some set of hosts and jobs. Jobs are submitted to a Streams instance, and the instance deploys those jobs to some of the hosts that instance manages.
Import/Export only works between jobs in the same instance; jobs in separate instances have no knowledge of each other. So part of your question is not possible: there is no Import/Export between jobs in different instances.
In case you meant to ask about the performance difference between Import/Export in the same job versus in different jobs, there is none. However, there is rarely a good reason to use Import/Export inside the same job, as the purpose of Import/Export is to enable communication between jobs.