Compression of already compressed files

Compression of already compressed files doesn't gain much.
So how about expanding compressed files prior to adding them to an archive?
Most important for me:
• DMGs can be compressed much better, when they are converted to uncompressed read only before compression.
• Most installation packages contain a gziped Archive.pax.gz - in general these packages are still working when Archive.pax.gz is replaced by the expanded Archive.pax.
The resulting archive of the package will be much smaller, (optional BetterZip might recompress the Archive.pax to .gz when expanding the archive)

Maybe dissolving a DMG container when compressing and restoring when expanding can make a big difference too.
The same idea could be applied to other types of containers (tar …), archives (zip …) or even lossy files (like sitx can compress jpegs by undoing huffman coding and restoring it on expansion)

Of course these "tricks" can be applied only when bitperfect restoration of the original files isn't the primary goal. Only the contained information is preserved perfectly, the representation can be slightly different when recompressing files.

For example I download a zipped dmg which itself uses gzip (not uncommon).
What I want is a 7z of a read-only-dmg (w/o compression)
In most of the cases I'll end up with a file at least 50% smaller than the original zip.

Regards Markus

PS: forgot to mention I use 7zip most of the time where recompression to 7z almost allways pays