I'm a little late to respond to the inquiries...I have an Yakima Rocket Box and have had it mounted to the factory cross members, but its really too big for the Sport. It doesn't allow rear cargo hatch to lift up all the way without the spoiler hitting the end of the box, even when the cross members are moved as far forward as they can go.
I'm certain that lifting it with Yakima's fixtures may resolve the obstruction, but honestly, I didn't want it on all the time. The Rocket Box is really really big. I really liked it on my Yukon XL, but...
Oh, and if you do buy one, plan on loaning to all your friends. That's my experience.