> This claim needs some real world evidence to back it up (and usually it's not about a performance impact, but instead a perceived image quality impact).

We're talking lossless compression here, so image quality is not the issue.

Fortunately someone else has already done this research. There's a tool for Windows to control the compact.exe behavior for individual folders called CompactGUI: https://github.com/IridiumIO/CompactGUI

They maintain a database of compression results here: https://docs.google.com/spreadsheets/d/14CVXd6PTIYE9XlNpRsxJ...

Reductions in storage use of greater than 50% are so common that they're hardly even worth remarking on. My experience with compressing a bunch of games is that the biggest gains come from compressing bloated asset packs. Hard to know what else could be taking up more than 50% of the storage space in a particular game.