The overdraw in the nontransparent parts depends on the rendering order, which is back to front. This does not fully prevent overdraw. For instance, polygons of concave entities can be overdrawn regardless of the sorting. For fully preventing overdraw you need to render the scene twice, as the Doom3 engine does.

However this only makes sense when almost everything in your level uses complex pixel shaders, which is not the case with most A6 applications.