One possible problem with this testing is since you use ip_hash in the nginx config and run the locust/siege/ab etc from a machine all the requests will be directed to a single nodebb instance. So having 2 nodebb processes does't help since all requests are coming from the same IP. You can confirm that by checking cpu usage in top during the bench.
Wasn't sure where to find the hash, but git put it in the package.json file. I did not alter this file or any other after installing 0.6.0, so I'm very confused about how this conflict arose. The conflicting line was the version number, which in my local file was, apparently, 0.7.0-dev (I believe). Again, no clue how that got in there.