Why would buffing do anything to the luster of the paint?
The paint has reached its maximum shininess, buffing it out wont do anything at all besides removing the surface of the paint.
Take a cloth, rub it really hard, look at the cloth, and the paint will end up on the cloth.
You want to sand it to fill up any scratches or rock chips if you want to clearcoat it. Removing dirt and scraches is what you should really worry about before you clearcoat it. Dont want a scratch or dirt spot under the clearcoat that you can never repair.