[pgeorgi]

Also if you have the same executable running several times on a machine, they share the RAM for the code (read-only pages, which should be most of them).

That doesn't work for UPX because each execution decompresses anew, which makes it a "new executable" from the OS' point of view. The only thing that would help is kernel space merging, but that's really only activated for some virtual machines.

[icebraining]

that's really only activated for some virtual machines.

And even there, it's a security threat, since it enables cross-VM timing and other attacks.