I was thinking along similar lines, bit it becomes a theoretical question when you cannot test/evaluate/interact with subjects anymore.

Also, I'm wondering whether a difference of 1 IQ point is even noticeable, and if not, what's the smallest noticeable increment or faction of a unit.

A difference in 1 IQ point in the 100-101 range might be a difference in absolute problem-solving ability of x units, while than the difference between 170-171 is y units.