What do things look like when zoomed in on the low drag region? A very good pilot can feel a 3% difference in airfoil drag. In my experience, ~3% is the threshold. We can add a lot of caveats to that ~3% ... but I won't.

Note in my polar plot above the Cd scale does not go to zero. In my plot I can see about a 5-10% difference between the top two at a Cl around 0.5.