How to set 7z compression level using GUI in Ubuntu?

My idea was to find some hint in the file listing of the packages or in gconf, but I had no luck. Someone else knew the answer though.

Superuser: Change default compression levels for file-roller?

Either:

$ dconf write /org/gnome/file-roller/general/compression-level "'maximum'"

Or:

$ dconf-editor

1. Select org in the left-hand pane.
2. Select gnome in the left-hand pane.
3. Select file-roller in the left-hand pane.
4. Select general in the left-hand pane.
5. Select compression-level in the right-hand pane.
6. Set compression-level to 'maximum'.

Unfortunately, you cannot fine-tune this setting for different compression programmes. If you want all the freedom, you can use your shell instead.

Image of dconf Editor with file-roller's compression-level set to maximum..

That answer previously had no upvotes, so I checked if this has any effect at all and choose to compress some documents with the default settings and with maximum settings (type 7z). Result: 2,3 KB improvement on a ~300 KB archive.

Caution:

• Depending on the content it can have a negative impact to compress everything with LZMA/LZMA2 on level maximum. PNGs for example are already compressed with Deflate and won't gain much from compression. It would make more sense to optimize them with PNGOUT/OptiPNG, Zopfli and then archive them with LZ4, achieving faster archive decompression and reducing file size.

I tried to find out more through file-rollers manpage and user manual, still no luck. Then I dowloaded the source package and searched for "maximum" in fr-command-7z.c (note how I avoid to say I read the source), which gave me the following:

switch (archive->compression) {
case FR_COMPRESSION_VERY_FAST:
    fr_process_add_arg (command->process, "-mx=1");
    break;
case FR_COMPRESSION_FAST:
    fr_process_add_arg (command->process, "-mx=5");
    break;
case FR_COMPRESSION_NORMAL:
    fr_process_add_arg (command->process, "-mx=7");
    break;
case FR_COMPRESSION_MAXIMUM:
    fr_process_add_arg (command->process, "-mx=9");
    if (! _g_mime_type_matches (archive->mime_type, "application/zip")
        && ! _g_mime_type_matches (archive->mime_type, "application/x-cbz"))
    {
        fr_process_add_arg (command->process, "-m0=lzma2");;
    }
    break;
}

That's as far as I can currently get, it seems like there is no ultra setting.