That's around 300 requests per second. Assuming 1000ms upper bound on requests, you need 300 workers to handle that load. Assuming 150MB per worker, that's 45 GB of memory required to handle the load. So like... 5 m4.xlarge instances on EC2 (to give redundancy and allow loss of 2 hosts). That's $700/month.
That's not that much. We've got a Rails app that pushes 5000 qps. And to be fair, we just dial up the number of instances and it handles it fine. It runs on over 100 instances. It costs us around $10k/month. Not the end of the world, cost wise, but we have multiple Go services that handle similar levels of traffic and run on a dozen instances. Additionally, deploys take a long time (rolling restarts plus health checks on 100 machines takes time).
Moving to Go (or Elixir) allows us to handle far more requests per unit of hardware. While latency would indeed improve, it's not the primary motivator for us moving away from Rails.
I haven't even mentioned the websocket story on Rails. That's a whole new can of worms.