Although you have to get the exhaust port so that you can see your face in the finish, you must first get the surface smooth and uniform.
The surface should be smooth with no whoops which will cause turbulence.
Likewise with the intake, all casting imperfections, steps and scratches must be removed, and the finish should be somewhat less than mirror finish, to retard the beading of the incoming fuel.
Long shaft cutting tools are a must, so that the business end can be supported close to the work area, otherwise the cutter can chatter, causing imperfections.