Realistically you wouldn’t be reading anything from the TCP buffer because you would have TLS between your app and TCP, and it’s pretty much guaranteed that whatever TLS you’re using already does buffering.
That's effectively just another small application layer buffer though, isn't it? It might shift what would otherwise be in the TCP receive buffer to the application layer on the receiving end, but that should be about all the impact.
Oh you’re right, I’m just so used to making the TLS argument because there is also the cost of syscalls if you make small reads without buffering, sorry xD