B+W XS-Pro Clear MRC-Nano 007 Lens Filter on a Nikon 16-85mm. It was the only photo to show the sun-spots out of many that were shot towards the harsh, cold, overcast morning sun. Yes, I know it's slightly out of focus and not the best exposure, but were talking sun-spots.
Would it have the sun-spots if the lens filter was not used? ... we'll never know.
Edited to add: I also own the 28-300. Informal testing with and without the same model filter as above did not show any visual difference such as sharpness, color, or contrast.