I'm not certain that Javascript isn't similarly `optimised' by such networks, but images are significant for the mixed content case.
I believe that we collected some data in Chrome about the efficacy of a content addressable cache in improving density that was underwhelming, despite expectations. I don't have it to hand, but I suspect that it'll appear in a paper at some point.
I've not seen data about how well it reduces network loads yet, although I believe the same colleagues were intending to collect that too.
The fact that JavaScript would break on networks that transparently rewrite JavaScript is a feature, not a bug. If hash codes were implemented by all major browsers, those networks which tamper with JavaScript would have to stop doing so pretty damn quickly, or face a lot of angry customers.
As it is, I think customers would already be angry if they knew that their network provider was rewriting code that will be executed on the customer's computer; anything which makes this more obvious can only be good.
I believe that we collected some data in Chrome about the efficacy of a content addressable cache in improving density that was underwhelming, despite expectations. I don't have it to hand, but I suspect that it'll appear in a paper at some point.
I've not seen data about how well it reduces network loads yet, although I believe the same colleagues were intending to collect that too.