The Betz limit is based on simple momentum theory, there are some that actually question whether it is possible even to reach it. As nobody has exceeded it yet there seems little point in worrying about it.
Commercial big machines claim a blade efficiency of about 45% with all the effort that is put into doing the very best with that level of finance invested.
Small machines typically manage about 35% without a lot of special effort. I have never seen any genuinely proven figures for much higher than this with all the clever profiles and twists that some seem to think necessary.
With a mppt scheme where the blades can operate on the peak of the curve I suspect blade design may be just a little more critical, my findings show that the best results are with blades running slower than the theoretical speed in higher winds. There is almost a doubling in output power with mppt compared with fixed voltage loading and that is s far bigger gain than any messing with the perfect blade is going to produce ( you are not going to see higher than 45%).
I have never seen any evidence to support the crazy angles and widths that the calculators predict and my experience is that they are a hindrance rather than a help except for machines with start up problems.
If you can manage mppt there is probably a case for trying to accurately duplicate a proven aerofoil shape, but for conventional fixed voltage loading I doubt that it is worth the bother.
As you say the problem of measurement is virtually beyond the average experimenter and what seems to be an improvement due to one change may really be caused by a change in another factor. Forget Betz, if you can manage 40% you have done extremely well.
Flux