Even without the flow analysis and better typing, incremental compilation is a huge improvement over TypeScript, which re-parses type declarations on every compilation. That quickly leads to large compile times when you have type definitions for third-party components (even though you may only be using one definition in the file, the entire definition is parsed).
The existing available definitions from the DefinitelyTyped project is a huge productivity booster. Apparently Flow also has similar .d.flow files, but it will probably be a while until they exist for common projects.
I was going to comment that this sounds like an opportunity to get mileage out of the huge amount of typeinfo already provided by DefinitelyTyped by building a tool to convert .d.ts into .d.flow files.
After investigating this thought, it looks like this already at the top of the list for future plans for Flow:
The existing available definitions from the DefinitelyTyped project is a huge productivity booster. Apparently Flow also has similar .d.flow files, but it will probably be a while until they exist for common projects.