Unicorn (3 workers) + Sinatra gets 2,300 requests per second.
Thin + Sinatra gets 1,000 requests per second.
https://github.com/joevandyk/unicorn-on-heroku for unicorn code/results.
https://github.com/joevandyk/unicorn-on-heroku/tree/thin for thin code/results.
The weird thing is I tried scaling up to 10 dynos (even tried 50). I didn't see any throughput improvement.