Well... new renderer and JS engine are faster and use less memory, which means less CPU cycles are used for the same thing and less information is stored in RAM, there are improvements for Windows and Linux hardware acceleration. This all summed-up means reduction of power consumption.
The new style system is faster, but partly due to parallelizing, which doesn't necessarily mean fewer CPU cycles (though _can_ mean more time spent in CPU sleep state).
I don't know why you think 57 uses less memory than 56 for the "renderer and JS engine". I don't think it does. In fact, the data shows that it actually uses a bit more than 56, though this obviously depends on workload.
(Disclaimer: I work on the Firefox rendering engine and have done some work on the JS engine.)
In theory, race to idle might save it. In practice, my fellow web app devs will throw more crap on the web page until things are slow again and only then push back on feature creep from management.
