The hidden partition, you may be referring to, contains the Windows 8 factory image.  It should be around 10 GB in size and can be removed if you create a Recovery drive, it will give you the option to remove it.  I would be certain I had the image somewhere safe before doing anything else.
I would also make sure I had a System Image Backup of my system in case something goes wrong.
Ubuntu 13.04 or later can be installed as UEFI if you wanted.  It is also compatible with the Windows 8 secure Boot.  Linux can also be installed as Legacy on a GPT configured drive if it was necessary.  The Details of how to do this will be up to you.
The key you recovered, as far as I know, can only be used for installing the original Windows 8 that came on the system.  It should not work with a downloaded version or purchased DVD, but I haven't actually tried it.
You should realize, if you remove the Recovery partition, you may loose any special recovery options the OEM supplied with the system.