On my wife's X3 we use Whisbar crossbars (very quiet) and a Thule Atlantis 1600. Wish we had gone one size up to the 1800, but I'm sure if we had, I'd wish we went one size up from that too
. With just the bars on, you forget they're there. With the box on, you'll always hear them, but the Thule isn't too loud at all. Even with the 1600 and smaller X3, we fit a family of 4, including 2 small kids with all the extra gear they require, without much issue for a week away at the beach. This includes beach chairs, umbrellas, toys, pack 'n play, portable booster seats/high chair, double stroller, diapers, etc (my wife does NOT pack light).
We're going back up in size to an X5 soon, but that's more about wanting more room for every day use than just for vacation. We'll need the box either way for beach trips where we need all the beach gear. Without that, we wouldn't need it.