rails-api is a sub-set of rails to be used when creating a JSON API app.
Dependencies shouldn't be an issue, since it's basically just a stripped down version of rails. Supposedly it is passing all tests. (Not sure if that includes testing with stuff like jruby)
To manage expectations, please see the policy of the ruby project for adding new packages: http://www.gentoo.org/proj/en/prog_lang/ruby/index.xml#doc_chap7