Tested it with a "rare" fast belt - item speed is exactly the same as the normal fast belt.
Not using any other quality mods.
Playing space age

Yes, that was a bug, if in your cursor lasts only 1 item, when you place it it wont change any stats. Now this bug is fixed cuz new mod version just dropped. If anything still not working let me know.

Mod broken.

It actually does work, you just need to enable the shortcut "Toggle belt conversion" => on.

Any existing quality belts you can convert or unconvert (to revert the entities back if you want to remove the mod) with the conversion tool shortcuts.