Indeed, both the the things that you mention would be very important improvements, but I am not aware of any plan at the moment:
AVX/FMA would be very nice, but at the moment, it looks like the browser world is still trying to ship "any SIMD at all", with the initial target of SSE2 still being in the future: it's only efficiently supported in Firefox Nightly at the moment. So it's probably a matter of, "one thing at a time". There might (or might not) also be a typical concern in the Web world, of introducing compatibility issues when some new feature is only well-supported, or only fast, on a subset of client machines. That kind of concern generally makes the browser world err on the side of being conservative. Which is a good thing in general, just not for us!