I would say that the shape of the LE is less important than the volume of the vacuum formed aft anything but a perfectly sharp edge.
That empty volume is what draws molecules of air into a turbulent flow within it, which cost extra energy.

Measurable?
Not really for our models and their speeds.

Observe, however, the TE of commercial and war planes, as well as the TE of any propeller.
They are as sharp as practical (there is a point beyond which the part becomes fragile or too flexible).

For some reason, whatever is easy to do, seldom has the best results.

I don't buy the "square for responsiveness" statement.