At the time I had also compared the standard incandescent bulb and the LED bulb for the rear fog light - but it was obvious that the incandescent bulb resulted in a brighter, redder, light. Hence I kept the incandescent bulb in place.
But, while working on final rear diffuser mod, I decided to see what would happen if I took the next logical step of installing an HID bulb in place of the incandescent bulb for the rear fog lamp. Here is the result.
So again the regular incandescent bulb seems to produce the most light for a red lens, as used in the rear fog lamp. I have to conclude that, despite the HID being "white" versus the yellow of the incandescent, either a yellow light produces more usable, visible light and/or for a RED lens, a whiter light behind a red lens does not always produce the brightest possible light. Or, perhaps an HID is directional and so works best if used in conjunction with a mirror lens surface that focuses the light in a proper way (like the headlights).
So the next step will be, researching the brightest possible incandescent bulbs! Stay tuned....