Every aero theory I have seen predicts that a sawn off shape (Kamm theory) is better than a too-round trailing edge. This is certainly consistant with the wind tunnel tests I have been part of too.
The theory holds up, as long as it is not "stretched" too far. At extremely low speeds, theory doesn't (at least shouldn't) be used to attempt to explain air flow. Once airspeed has reached a stable point then the theory is pretty good. Of course at foamy speeds, as you well point out, it really shouldn't be applied. At speeds of 15 mph or more, it holds very well.
There are, I will grant you, a lot of folks who quote theory out of context and without full understanding. You can push anything too far and, if that is your point, I will agree with you.
On the other hand, I have seen many many times where low speed flow, once stablized, is essentially the same as higher speed flow, even with regard to separation points, laminar vs turbulent flow boundary layer, and so forth. When we would run flow visualization on a car, the ink would flow and the tuffs would blow at pretty low speeds.
I am not speaking as an aerodynamicist but as someone who has still spent maybe 900 hours actually working in a wind tunnel. How much tunnel time do you have?
Allan