Java went through a similar issue with their pack200 format. I am not privy to where all the time is spend in mobile JS startup (parsing, etc). It seems like using some kind of special JS precompressor would both benefit entropy coding as well as allow a faster parser, but I've seen it turn out to be a wash in the past. :( However, even if you could reduce network bandwidth, it would be a win for people's data plans and batteries. :)