I painted it semi-gloss. To me rubber often is not totally flat. However, that extension does not look that different than the gloss of the filter, so I may indeed repaint that area with flat.
Another painting problem. The bodine cables (like for brakes and throttle) are black vinyl tubing. However, it looks like on the real bike they are sort of a metallic gray. I have never been able to paint that soft vinyl. The paint never hardens. I tried a marker, that is not doing it either.
I built that kit a couple of years ago. You are correct that the cables are gray, not black. I was fortunate to have access to an original bike for reference. Light Gull gray was the best match.