We have a new version of guetzli with two improvements: new more precise butteraugli and smarter handling of low frequency features. New butteraugli gives possibly a 2 % size improvement and an unspecified improvement in quality, but makes guetzli twice as slow and to use even more memory. The improvement in low frequency modeling possibly fixes guetzli to actually work well at lower qualities, at least down to 84. More experiences are needed of both before large-scale deployment...
https://github.com/google/guetzli