I've been using babel instead of the internal transpiler and yes you can use all the ES6 syntax out of the box. I think the JSX transpiler was deprecated some

I've been using babel instead of the internal transpiler and yes you can use all the ES6 syntax out of the box. I think the JSX transpiler was deprecated some time ago and pretty much all the react starter templates and examples have been using Babel. I don't think adopting babel is going to compromise your use of coffeescript, in then babel is just transpiling down to old style javascript.