In general it just treats the instances as an additional mesh to render, so performance would be similar with the same number of polygons between an instanced model and non-instanced model.
Prefab will share texture memory between instances, so that will help GPU memory. In general, what is your concern? CPU memory? GPU memory? Performance?
I see - it won't change that. If we need to draw a poly, there are not many ways to improve that, except by batching. In this case batching for a particular model is most dependent on texture changes, so if you use a texture atlas / single texture for a model, it will be able to batch well.
