5% reduction in milage if you use the square roof bars, plus the noise of square bars is annoying. I now use Rola's aerodynamic roof bars that fit between the existing roof rails on both my Subaru Impreza and Mazda 6 station wagons. Quiet, no significant reduction in milage, look good.
Hauling a plastic 1+1 sit on fishing kayak. My parents have a two person inflatable. Inflating isn't too hard, but speed an manuverability on the water is sufficiently impacted that I'd recommend a hard shell.
I have brackets on my garage wall that hold the kayak and windsurfer.