I routinely shoot my Portra 400 as I said above, which is really shooting at more like 200. For Portra 160 I usually set the meter or camera at 125.
Color neg is almost always better with a bit more exposure. ISO is a carefully defined speed point, but that does NOT mean it is the optimum exposure index for best quality. For that matter I expose most black and white films at lower EIs than the box ISO speed too.
"Best overall results" will honestly be obtained with Portra 400 at something less than box speed. Same with almost any C41 film. That doesn't mean box speed isn't good. It is.