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Originally Posted by Lil bastard
Good points!
The term 'polishing' is really a holdover from bygone days. Back then, it was thought that smoothly polishing the walls of intake runners produced less turbulent airflow.
In fact, computer modeling and real-time flow bench testing has confirmed that you really want a finely roughed surface, similar to that of a frosted glass. This is how F1 engines have been 'polished' for better than a decade.
This finely roughed surface better supports the establishment of a boundary layer along the intake runner walls allowing air to flow more smoothly over it.
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What the rough surface does is encourage a turbulent boundary layer, as opposed to a laminar one. The turbulent boundary layer is typically thinner and less "sticky" to both the bulk flow and the wall, helping the bulk flow.
In the case of mold flashing, however, if it's there, its bad for flow.