Here are some actual numbers for you, the size of the ps2packer-compressed OPL ELF:
680637 bytes with the original PNGs
679357 bytes with uncompressed PNGs
670365 bytes with optimised PNGs
So the version with optimised PNGs saves just over 10KB vs the original.
Updated optimised and uncompressed PNG archives are attached. It turns out Open PS2 Loader's PNG-handling code doesn't handle paletted images where one colour is transparent. It needs all images to be RGB + alpha.
Note that when I used PNGOUT to generate the uncompressed PNGs, it strips out unneeded tags like file creation dates so that will save a little space. We can see that ps2packer-only compression does work out slightly better than the original not-optimised PNGs. But the optimised PNGs are significantly smaller than that.
Generally, when compressing data you get the best results applying the best/most suitable compression method first. In this case, the optimised PNGs are compressed better than ps2packer could manage on the uncompressed PNGs.
Using AKuHAK's example, you get better results compressing with 7-Zip first and then Zip, instead of Zip followed by 7-Zip. (Obviously there's no point in compressing with the worse Zip compressor after 7-Zip, but in some cases you do end up compressing some data twice, like here with PNG compression and ps2packer run on the entire ELF file.)