I tried the head rest on my NL 8x42 and you are correct. For me anyway It reduced some of the bigger shakes but it didn't get rid on those micro jitterrs that IS removes and those are the ones that are really bothersome. The head rest did not compare to the IS on my Canon 12x36 IS III or Nikon 10x25 IS as far as reducing small shake and movement. At least for 8x I don't think the head rest is worth it.While the head rest on the NL will surely be somewhat helpful, it is unlikely to be as effective as a stabilized glass. The joy of the stabilization is that it removes the micro jitters inherent in hand holding.
Those are not removed by an added rest, whether on the forehead or on a finnstick.
At least in my experience, even if resting my Canon 10x42 on my glasses while looking at a bird high above me, engaging the IS greatly improves the ability to discern the details.