I have been digging in my iPhone 4 and what I found is this :
These files are located in : /private/var/mobile/Library/VoiceServices/Assets/*language-code*/
Yup, the FULL versions are used in iPhones and iPads.
Obviously you can't really obtain them from decrypted ipsw image. This one contains only support files, which can improve TTS experience. ATM I found new version of Agata voice better than GPS-based one. I haven't used support files like dictionary and new config files yet.
Cheers