I think I finally find out (but can't confirm myself at the moment).
Looks like Lumia 830 was released with a half baked Denim Firmware Update.
To be allowed to install Lumia Camera 5 on Lumia 830 it need to be updated at least to O.S. version 8.10.14226.359 currently unavailable even trough Dev Preview.
Yesterday my phone finally receive an update as you can see below:
It said that the phone would update this:
- Windows Phone 8.1 (8.10.14234.375)
- Lumia Update for Windows Phone
Even though the version is above the one I report this update made my phone fully support the new Lumia Camera 5.
One more think: I needed to disable Dev Preview again to see the update, something that I've already tried with no success.