UPX does significantly more compression on binaries than gzip can on the layers. It's not just simple compression, it makes a lot of modifications to the binary to make it more compressible.
Perhaps this is what you were asking for in the GP, but it would be interesting then if UPX could apply those transformations without actually doing the final compression step itself. Perhaps the win isn't as great without the ability to smuggle specific hints to the compressor, but it would be interesting to see.
On the other hand, if it could be demonstrated that modest increases to binary size correlated to significantly improved compressability, then maybe that's something that should just be considered by the compilers themselves.
