oh I don't doubt it makes a difference, but I bet outer flap outside the zip like parka or aviation jacket would work better against wind and water creeping in, now it won't look as nice perhaps could not have 2 chest pockets unless the zip is made off center to make the outer flap look centered.Theoretical thinking is one thing; practical experience with them is another. Wind flaps work. As you live close to the equator, you will, fortunately, never have to try one out.
While wind flaps are essential on motorcycles with no windshields, I find they are helpful on motorcycles with windshields as well. On both the fairing and non fairing models I have had, there is enough wind that gets around the windshield that I want wind flaps on all of my riding jackets, especially when the temps are in the 50s and below.
my argument was about inner flap vs outer flap
not with vs without flap