This is extremely cool. Someone needs to do this for all the languages that are commonly used. Knowing general speeds of various calls for javascript, ruby, elixir, etc. would be great for web development.
There's a number of benchmarks at [1]. It would be nice if somebody would compile+run them on an AltJS environment and publish the result for different browsers.
I do not think that it is that useful. While i have seen people greatly overestimate computing complexity of simple tasks like comparing numbers, i do not believe knowing how long such instructions take gives you much more. Especially interpreted JIT compiled code is not exactly predictable in performance and optimizing single instructions not very useful. However it is a fun game and does indeed give a better rough assessment for performance in various implementations. But if you really like to optimize your code these are just some hints. You will have to run benchmarks for every use case.
A high-level look at the relative speeds of common JS operations and patterns would be very useful. I often wonder if I'm wasting time thinking about how to optimize something when it won't make any practical difference.
[1] http://benchmarksgame.alioth.debian.org/