A useful way to think about throw, which I haven’t seen talked about in this thread, is I=LA, where I is the luminous intensity of the flashlight, in cd, L is the luminance of the LED, in cd/mm^2, and A is the apparent area. I learned this from Dr Jones thread on this subject:Flashlight Optics - Dome, Dedoming and Throw
When you hold the flashlight at arm’s length and look into the head and see the yellow color of the phosphor filling the reflector surface, that is the apparent area in the above equation. An OP reflector will generally reduce the apparent area. If you look very closely at an OP reflector, you will sometimes see that the yellow phosphor doesn’t fill the area uniformly; it is not fully saturated with the yellow. Some of the crests and ridges in the reflector are not reflecting the LED surface.
Djozz’s test results quoted above are interesting. The result was that for small LEDs an OP reflector significantly reduced throw compared to a smooth reflector, but there was not much of a difference in throw for a large LED.
With the above ideas in mind about apparent area, my hypothesis to explain djozz’s results is that for the small LED, the crests and ridges of the OP reflector changed the effective focal point of the reflector enough to “miss” the LED surface. This would reduce the apparent area. But for a large LED the deviations were not large enough to “miss” the LED surface and the apparent area stays intact.