Is this a good idea to pack all the executables with UPX? Of course, except of busybox & shell scripts. And this will need "hack" md5 in filelist of packages. The main goal is reduce RAM usage by system. Will it work?
Nope. It will decrease hdd usage, but not RAM. In fact, it will use more ram: >Currently, executables compressed by UPX do not share RAM at runtime in the way that executables mapped from a filesystem do. As a result, if the same program is run simultaneously by more than one process, then using the compressed version will require more RAM and/or swap space. So, shell programs (bash, csh, etc.) and ''make'' might not be good candidates for compression.