I must admit confusion to what you're trying to say. You called BS on it, then say you know the science, but then you deny the science.
The testing wasn't done by the people buying this, it was done in test labs before it was sold. Are you calling the manufacturer a liar? That it despite the science, you think it shouldn't work? That you don't understand it? It isn't worth it? Anyone who pursues a couple of knots of speed is a fool?
There is such a range of things I could read into your posts.
Trying to be less wordy. But since you asked:
What I get the science behind:
-Premise 1: That VGs work.. obviously. They're used in various applications to energize airflows, etc. So I "get" all that.
-Premise 2: That the propeller is also a wing
-Conclusion: That putting VGs on a propeller, in theory, could help the propeller operate more efficiently
What I am dubious about:
-just because the science supports it doesn't mean that this particular application will work as advertised. We get the science behind many things, doesn't mean we're able to put them all into practice
-generating more lift, to make the plane go faster, would impart a higher load on the engine and decrease your RPM (at static sea level).. no? Someone else said that the idea is to kill the drag from the parts of the blade not generating lift, that's how they get 50 RPM gain.., which in turn makes the plane go faster since the lift generating parts go faster.. but don't most people pull their power back to a set POH cruise rpm setting anyway, or do all these users just elect now to cruise 50 RPM higher? If they're going to set the prop to their standard 2500 (or whatever) then we're expecting a ton of gains from a piece of tape that at the same rpm the prop is that much more efficient to have an appreciable gain in airspeed
-1-2 knots in our planes is hard to measure. In the 172N (180) I used to fly you'd see anywhere from 112 - 117 KTAS depending on several factors.. these would be at your usual VFR 6,500, of whatever
-the engineers who build our machines are smart people.. if it all took was some striations on the leading edges of the prop in various areas don't you think this would be done? The most advanced, modern, composite props don't have this feature. You don't see any commercial turboprops with this either... where being able to sell this kind of efficiency gain would actually be very valuable to all the Dash 8 Q400s flying around
-when asked once (I heard this like fourth hand.. so tremendous gain of salt) why Airbus planes don't have all the little VGs on their wings that some Boeings do.. the response was that a well designed airfoil shouldn't need them. Maybe this was tongue in cheek, but there's merit to that
-the propeller operates at a very wide range of angles of attach.. are these VGs optimized for low AoA (cruise) or high AoA (climb).. I doubt you can do both
-not saying they're liars.. but the poster asked if we thought this was a gimmick, and I'm in the camp of it being a gimmick. Most people who are putting $200 tape on their props will likely be confident they felt a gain from it it as well