It's not just about string performance, it's about making wasm a first class experience on the web. That includes performance improvements - because you don't need to wake up the js engine - but it's a lot more than that. Including much better dev-ex, which is not great as you can see in the OP.
It would also enable combining different languages with high-level interfaces rather than having to drop down to c-style interfaces for everything.
It would also enable combining different languages with high-level interfaces rather than having to drop down to c-style interfaces for everything.