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If I understand the question, for improve the surface finish metalic mould is better than sand mould. Inclusive you can polish the surface of mould like a mirror.
In relation of the fluid of material, It can be improved with the aggregate of Silicium. With this element the filling of the mould is better. The percentage of the eutectic is 13%. In this case, the problem is that the solidification occurr in instataneous manner. For this reason you can reduce the %Si in order to have the material in pasty form, and improve the die filling and reduce the part defects.
For the surface finishe, the permanent mold may be a good choice. Generally, Al-Si have a higher fluditiy than other alloys, you can modify the Si contents to achieve your target. For the microstructure, the high quality of liquid, especially reduce the oxide file, will help a lot to remove the defects during casting.