the roughness of the aluminium of the nose cowl would be below the boundary layer in most circumstances. most of the surface conditions would be a result of the tooling used to manufacture the part. any turbulence caused could actually improve the aerodynamics of the airflow into the cold stream duct,well, until it passes through the blades themselves.
as for effecting stall speed? i seriously doubt it would make any difference.as for corrosion damage, the SRm would state tolerances and minimum thickness of original material before it requires repair.