Fuji NPH rated at 250 is excellent for portraits. As for higher speed film, I love Kodak Supra 800 and I rate it at 500-640 since it does not tolerate underexposure at all. Fuji NPZ (800) is a superb film but you have to import it - the local shops dont stock it.
It's only to use the film at it's "correct speed". A lot of times, ISO 400 film is not really ISO 400, but ISO 250 or 320. By using it at ISO 320/250, you get a more correct exposure. Also, for negative film, underexposure is very bad, but negative film can handle quite a bit of overexposure. Setting a lower ISO can help prevent underexposure.
That said, it's still important to get your exposure correct, whether you are re-rating your film or not.