I’ve been excited about asm.js too! I recently used it on http://zenphoton.com with conditionally-awesome results.
The spec is really intended as a format for automated tools like emscripten, but it’s not that bad to write it by hand if you have only a small inner-loop that needs optimizing. I chose to write most of my app in CoffeeScript, with a small computation kernel in hand-written asm.js that runs on a Web Worker thread.
It’s great! : ) I’ve tried it on Firefox Nightly and on Chrome, but I couldn’t appreciate any performance differences. Are you using the non-asm.js version of the renderer?. In any case, I love it: it is brilliant (he, he).