The binaryen toolkit comes with a wasm2js tool, you could translate the wasm back to js and see how performance compares ;)
It's possible that performance isn't all that different, because asm.js-style Javascript can be surprisingly fast (compared to "idiomatic" human-written Javascript).
Otherwise it's a completely pointless excercise of course, unless you need to support browsers without WASM support (which don't exist anymore AFAIK).