Imagine you had the right amount of space to achieve what you are describing for a skinny foot. Then as the boot dynamically flexes in certain snow conditions, that space becomes larger than desired, right at the time when there is the most potential energy in the interface between your skinny foot and boot. Perhaps this is a poor analogy, but it like the last suddenly goes from 98 to 100, right at the worst time.
One thing I like about the X-MAX is the way it wraps my foot, and that wrap remains consistent throughout the range of the boot's forward flex. If I understand you correctly, that is a bad thing?