There are a number of scripts/programs out now which combine multiple png optimisation tools in an attempt to get the best result. For example:
Article about performance on various tools here:
It is in French but luckily they use Hindu-Arabic numerals, so the numbers can speak for themselves.
I appreciate you looked at optipng before and rejected it through too much overhead (https://sourceforge.net/tracker/?func=detail&aid=2018796&group_id=212588&atid=1022338).
However would it be possible to have an option allowing for the optimisation of gifs → pngs, the crushing of pngs & the palette reduction of png32 → png8 should it be so desired?