I generally wear a plate carrier, but I don't put plates in it.
Due to the simple fact of the way the joints in the human arm are put together, having your mags a bit lower down around the stomach or belt area is generally fastest and most ergonomic for most people. Conversely, the closer a pouch is to your chin, the harder it is to draw from; that's always been my experience anyway. Whatever's most efficient for the wearer.