In the app/config/unicorn.rb we changed the backlog line to this:
:backlog => Integer(ENV['UNICORN_BACKLOG'] || 200)
And then we can alter the backlog as necessary via:
heroku config:set UNICORN_BACKLOG=25 -a <app_name>
We found 25 to be a sweet spot for two Unicorn Workers but it may be different with four (which we are experimenting with now). Again this is also relative to the app so your results may (and probably will) be different.
I neglected to mention that you have to move the port declaration from config.ru and put it in the unicorn.rb too. The full line should be something like this:
listen ENV['PORT'], :backlog => Integer(ENV['UNICORN_BACKLOG'] || 200)