Oops, should have asked an engineer or someone who has read a book.

All castings contain some porosity.
If you cut a section through a casting and polish the surface to a 1 micron finish and put it under a microscope you will easily see the porosity wherever it intersects your polished surface. The metal is only in danger of falling apart if the stress is too high, or if there is so much porosity that the metal looks like swiss cheese.

The porosity doesn't help to transfer heat. The little air pocket inside the porosity is a pretty good insulator compared to the aluminum matrix.

Painting a metal surface can prevent heat transfer (and raise the temperature of the part), because paint doesn't conduct heat as well as aluminum. On the other hand, a dark colored paint can radiate heat better than a bright metallic part. In the temperature range below 400F, radiation of heat isn't very important compared to convection and conduction to the airstream.

If painting the cylinders really raises the engine temperature by 40F, 3W would be smart to quit offering the fancy paint option.