This needs an update thanks in part to this comment, but also because upon further testing tonight it appears Windows 8 simply couldn't understand me saying "Power".
When using speech recognition in Windows 8 the fastest way to shut down your PC that I now know is this:
"Press, Hold Windows, I"
"Power" (Accessibility hooks are present and correct, but recognition required further training)
This still leaves the issues of an incorrect, outdated and misleading tutorial; breakage of the "See and Say" model (due to the required hidden menus); and the need for mysterious undiscoverable keyboard shortcuts. This final sequence is however far shorter and more manageable than my earlier attempt, so I'll document it here...