Good points. However, regarding performance, you can get a lot of mileage by moving over to JRuby + Torquebox (cluster) and further improve response times by ad

Good points. However, regarding performance, you can get a lot of mileage by moving over to JRuby + Torquebox (cluster) and further improve response times by adding a caching layer and maybe throwing in Angular.js / Backbone.js (templating moves to the client side). So the goodness of Rails dev speed and the performance of JRuby is a decent combo IMHO. But like you mentioned, if you need to scale beyond that Scala is a good option.