Don't forget about PNG8 w/alpha.
Unfortunately reducing the color depth is not something Construct 2 can do automatically. Construct 2 must always work in the general case, and many images look terrible after color depth reduction. So that's not something we can always apply to reduce filesize.
We might be able to add compression settings in future, but for now everything has to be PNG32, and as you saw the savings for that are only around 12%. If you are that concerned about reducing filesize after export, you can run your own tools over the exported PNG images, and you should see similar gains.