How does the shader cache compare into one created by just playing the game? Does it just brute force every possible variant producing tons of compiled shaders not actually used by the game or does it have a way to avoid the combinatorial explosion? I guess the record option could help, but then you would need to play the game with stutters first to gather the data and it would help only if you need to recompile after that.

Of course, but I meant Valve could be willing to help GOG to integrate Proton into Galaxy if they start contributing to development.

Galaxy is just for automating installation and updates and implementing cloud saves and achievements. The installation is identical to one made with standalone installer and can be launched from the executable without running Galaxy client. Already installed games can be imported to Galaxy library.

Proton is kinda tied to Steam, but UMU already replicates those dependencies so you can run it outside Steam client, so GOG could do something similar and I doubt Valve would object cooperation if GOG wants to participate developing Proton.