I once hauled a small cedar canoe (12', 30 lbs) on a small Mitsubishi with a roof about the size of the Xfire and it worked fine. I wouldn't worry about aerodynamics if you tie it down well. As far as rubbing the bumpers, unless this kayak is terribly short, it will probably overhang the car on the ends and never touch the bumpers. You can always pad them with something anyway.

I would invest in some of those dense foam cradles to protect your roof and keep things better aligned, or at least a big foam pad of some sort. If you are a worrier, you can always strap your craft down fore and aft, then open your windows and run a strap through the cabin, over the roof and kayak for insurance.

And don't forget the pictures.