This could actually pretty damn cool as a method of using ES6 features and supporting browsers that don't yet offer it. Of course at a serious performance penalty I'm sure.
If you don't need to dynamically evaluate the code, compiling ES.next code will be more performant, e.g. using Babel.
Yeah, it would be slower. But you could use some kind of JITting while the interpreter goes and create appropriate ES5 code. That's possible next step.