I was hauling around some old wheels in the Focus with the seat backs down. I never noticed how much more noise you get with the seats down! I have studded snow tirs on mine, so it's going to be noisey anyway.
The other thing is, I would be worried about the carpet keeping in salt spray. That's going to rust the car out quickly in the salt belt
2011 Focus SE 5 Speed 36k - Blue Flame Metallic - Added Rear Swaybar
2000 Jeep Cherokee Sport 115k - Lifted with mud tires.