This is based on the assumption that you can apply different compression levels to different files within an archive? (that's possible right?)
It would be great if when creating an archive, there was a tick box option called "Don't compress compressed formats" or "Skip compression on compressed formats" or something like that.
This would by default compress all files using the user's select compression ratio. But well known compressed formats such as "zip 7z mp3 jpg avi mpg etc..." would simply be stored in the archive uncompressed as if it was a "Compression level: Store" archive.
This would save huge amounts of time when creating large archives that contain both compressible and fairly uncompressable files.