Thanks for posting - I hadn't really considered Chesterton's Fence as a technical metaphor but as soon as you posted I immediately thought of a person who has a habit of this very act. I'm adding this term to my lexicon.
Here's a gist of our unicorn.rb config (https://gist.github.com/apangeajwrubel/4953849)
Using env variables lets us adjust parameters without a code push (still requires a restart). We saw dramatic reduction in 'long tail' requests when we dropped the backlog to 25. We're experimenting now (thanks to @scottshea) with even lower values. At some point the routing mesh will give up retrying and throw an H21 (https://devcenter.heroku.com/articles/error-codes#h21-backen...). One data point that would be useful from heroku is how many retries we get.