Heads up:
The MediaWiki core change https://gerrit.wikimedia.org/r/#/c/119661/ for https://www.mediawiki.org/wiki/Requests_for_comment/Reducing_image_quality_f... being rolled out.
The next step we think is to rewrite the <img> tags on a popular page (e.g., http://en.m.wikipedia.org/wiki/Cats) when Wikipedia Zero is in force in order to give this a run in production.
We'll share findings once a patch is written and implemented taking advantage of the image quality URL rewriting capability.
-Adam