Cold crashing just means putting your beer someplace cold to help drop yeast and other particles out of suspension. By doing this you typically end up with a clearer beer. Most people just put their bucket, carboy, or keg at fridge temps for a few days.
If your porter has only been in the primary for 16 days I'd let it sit longer before doing anything with it. Maybe another week or two. Bigger beers tend to benefit from a little longer primary and some bulk aging.
But if your porter is already going to be completely opaque you probably won't notice any benefit to cold crashing it.
EDIT: Oops, someone beat me to the punch while I was typing.