Sure paint it, why not? People paint everything underwater with bottom paint, except anodes. I have 2 hard rubber underwater exhaust extensions painted with antifoul. These are real old, from the late 1960's and are in good condition.
The purpose is to send exhaust gasses underwater, make less exhaust smell.
They do modify the exhaust sound, although I dont know how many have any such thing anymore.
I suppose they also help a little when backing up, less water forced up the exhaust.
The bottom edge is right at level with boat bottom, so they can not catch on anything in the water.