racing with V8's we found the cast manifolds as manufactured with slightly rough surface performed better than hand polished glass smooth. This was due to the fuel air mix not able to stick to the walls and the manifold flowed better. That was for a carburated engine. For fuel injected at the cylinder entrance the condition was the same, cast surface better. We did however, match the manifold ports to the head port to remove any mismatch which is definitely bad for performance.
Of course all engines are different and for us track time and ET were the measure.
