Maybe we're thinking of different things here. What I had in mind was more along the lines of for instance choosing Go or Scala instead of Rails or *.js and getting a 10x speedup at the cost of simply adding a few type declarations. There are situations like yours where deadlines leave little choice, but there are also situations where a bit more leeway is available.
If you're founding a startup in an emerging consumer market, you'd be making a mistake to use something like Go or Scala rather than Rails or Node.js, because your whole success is dependent upon finding out what particular combination of design, features, and experience emotionally resonates with fickle consumers' minds. That takes a lot of trial and error; anything that slows down that experimentation process is going to cost you the market. Once you've found the market you can get VC and hire experienced technologists to curse out your technology choices and rewrite it in Go or Scala or Java or C++ or whatever.
If you're CloudFlare and billing yourself as "the web performance and security company", however, building out your architecture in a performant and reliable language makes a lot of sense. You know your value proposition: you want to use whichever technology stack lets you execute against that value proposition most effectively.
Twitter got a lot of flack for being built on a "dumb" Rails architecture, but I'm also certain they would not have succeeded had they done anything else. Remember the context of their founding: Twitter grew out of an idea lab that grew out of Odeo, and at the time of their founding they were an idea that was so marginal that nobody would've bothered with it had it taken more than a weekend or so to prototype. When it turned out that people liked it, then they could afford to hire people to rewrite the software into something scalable - but those people would not have jobs had the initial concept not been proven out first.
I work with both a RoR code base and a Scala code base. There is no difference in productivity that I can tell between the two. I think it is a out-dated assumption to believe that dynamic languages like Ruby or Python are more productive than modern statically typed languages. The only cost is one of learning; many more people know Ruby than Scala. This is an artifact of history (i.e. Ruby is 10+ years older than Scala; crappy CS educations that only teach Java).