Matthias Felleisen, one of the developers of Racket, recently gave a talk about the motivation behind typed Racket; it can be found here: https://www.youtube.com/watch?v=XTl7Jn_kmio
I wonder if you could do something like prototype in dynamic (original, if you will) racket and then release in typed racket for performance/safety reasons.
I think that was one of the purposes of Rackets typesystem, to fit idiomatic Racket code well enough that you wouldn't have to rewrite most of it to introduce static typing to a Racket program.
Excellent talk. Nice to see it a Clojure conference. The points near the end about contracts vs. types was enlightening.