The other thing to keep in mind is that all the air is not moving at the the same speed. If you plot gas velocity against port location, it's traveling fastest in the center and slowest by the port wall. At the point that the gas is in contact with the port wall is the boundary layer, and the gas flow is close to 0. This is the where the firling would be, so it would have almost no effect on the direction of air flow.
Now, one of the reasons for not putting a mirror finish on the port wall is it makes the boundary layer thicker (than having a slightly textured surface which breaks up the boundary layer) which causes more drag.
