I hunt in a ghillie quite a bit, and although I won't turn down the use of the full thing, you are definitely correct, my buddy and I both have worn just the head cover (in both cases, boonies with lots of shoulder) and both noticed the difference in being noticed.
It is said in some bio classes that prey animals (all critters really, including us) have an ingrained ability to pick out faces and heads. That's what keeps them alive.
In my mind, it does work, very well, and when moving, I often wear the head cover.
ChuckC