If reducing the mip level for the selected ergs significantly reduces the rendering time, then texture bandwidth may be a bottleneck, and if the change in visual quality is acceptable, you can gain speed by using a smaller texture.
If there is little or no visual difference in the scene when using a smaller mip level, then you are wasting texture bandwidth for this scene, and you can regain some texture memory by using a smaller texture. This usually happens when geometry is rendered smaller than expected. The hardware (in standard filtering modes) always chooses the best match for the screen pixels being displayed. So, the top-level mip will not be used if the geometry being rendered is significantly smaller than the resolution of the texture.