Interesting experiment getting rid of transpilation. No way we could do it for Superpowers anytime soon since we're in browsers a lot and have to browserify so much. But I'm excited for the day where ES2015 modules + HTTP2 makes it possible to do a lot of raw stuff again. Though I love them types too...
Have you tried using watchers for quick iteration? I don't know if babel has any, but TypeScript does tsc -w and browserify has a watchify counterpart, I think they are pretty fast. Still, it's more stuff to run and keep up-to-date...