My conclusion is that you can save some time, but in any case, you will have a lesser quality in shadows and reflections (if the light are soften, reflections are blured, what is better). An other thing to know is that if the picture has no reflection, you will not save a lot of time (even if the image has transmitted light materials) as shown by the table. An other strange thing is that the sky is always better in ultra mode. So for me, ultra (or user settings) is better in all cases.