Reusing HTTP connections is not relevant if the app is behind a reverse proxy like nginx, which, last time I checked, still didn’t proxy with http2 multiplexing connections to the app server.
Connection reuse matters in high latency situations like for end-user connections. The fact that connections over localhost (or even UNIX socket) between your reverse proxy and app server aren't reused is no big deal as it takes less than a millisecond to establish a new one anyway.
Depends where the bottleneck is. But basically, we're saying different server setups are also a factor and another reason why you need to do testing with representative loads :-)