You can hit endpoints / NiFi REST API with JMeter.
Check out this:
In the second link you provided, it says "We plan to provide a benchmarking and performance test template to include in the build, which allows users to easily test their system and to identify where bottlenecks are, and at which point they might become a factor. This template should also make it easy for system administrators to make changes and to verify the impact."
Do you know happen to know how far away this is? It's exactly what I'm looking for.