We ported a pretty hefty application to Heroku, and surprisingly, didn't have much customization that needed to happen in regard to gems. We did have a hand written Nginx module that we had to toss out, and took care of the stuff we needed to take care of on the Rack level. The Rack middleware we had to write was really minimal, and took only an hour or two to write and test. The application gets around 20-60 rq/s on Heroku, and we haven't had to touch a thing since the migration (/me knocks on wood).