So, sanding doesn't help for the reasons you think. The bond is at the molecular level, so scuffing the case doesn't "give it something to grip". What it will do is forcibly remove any contaminates, coatings, or anything else that might get in the way of adhesion. It's best thought of as cleaning, but mechanical rather than chemical.
That said, I wouldn't sand textured plastic cases, unless you're really going to town and making it smooth. Sanding will only hit the tops of the "bumps" of the texture. Clean the thing really well, and use a good plastic primer and you can put anything you want on it. The G81 case I painted got really nice things said about it at the recent SF meetup. That was just cleaned, masked, primed, painted with a MTN 94 product, and then a matte clear.
One consideration for a black case is to get a good opaque primer. Putting a light color over dark will require many coats.