Otherwise, if it's really relocating binaries, then it's awesomse, BUT I definately want to know MORE about the 'caching' part. That comes short in the docs.
Which bit of the caching isn't clear? If you're happy that whack creates relocatable versions, then the caching works just by copying the output directory into ~/.cache, and then copying that directory to the target on subsequent installations. The exact details of how programs are made relocatable are under "How does Whack work?" in the README.
Oh, no I hoped that something performance related is happening when you said cache, like tuning the binary, predictive read-ahead streaming of the binary to memory, or something like that. I didn't know that it's a copy operation.