In reply to Scott McRae:
I enquired about something similar (stretching across the width) from a couple of professionals and was told that it just isn't possible with modern mountaineering boots of that type as they will just basically return back to shape once the stretching device is removed. It's only really possible with all-leather traditional walking boots. Modern synthetic boots, especially winter B3 winter boots, are made to be solid, stiff and not to deform. Does make sense I suppose.