[rb2k_]

1-2% might be possible by using aggressive gzip encoders such as https://code.google.com/p/zopfli/

( as seen in https://github.com/sayurin/optipng-zopfli and mentioned in https://twitter.com/pornelski/status/356843309118922756 )

[kraken-io]

We have been experimenting with Google's Zopfli but found out that the optimization time is too long for such a insignificant optimization gain.

Optimization speed has always a key factor for Kraken.io.

[depsypher]

Another option using zopfli is pngtastic (disclaimer, my own project): https://github.com/depsypher/pngtastic

It really is significantly slower, but if you want to reduce every last byte in a png zopfli is your best bet.