This script works alright and does what it advertises with the current iteration of the Google Docs site. I tested several others that are not working these days.
The only drawback it seems is that there isn't an easy way other than enabling and disabling the extension to turn off the rendering change. This makes it a bit awkward to get to the stuff in the header when you do need it (sharing, doc title, etc).
Besides an enable/disable keystroke or other switch, it would be nice to have an option to hide the toolbar as well for a true full screen experience.