Try other frameworks like Play, Spring,before you are confident to speak out the word "productive", try other frameworks from other languages like Rails before you are confident about start up speed
I would say that's a great work. But, in order to write efficient code, you still need to understand both languages. It just saves you time to remember JavaScript syntax. Since we have google and stackoverflow, people can pretty much write more idiomatic code than machine. So the conclusion is not worth it.