I started with the Q-tower rack and found the curvature of the boat will push the towers apart when I cinch down on the boat, regardless how tight I install the towers. I end up getting the Yakima tracks and control towers and drilled the tracks into the roof with the help of a friend 5 years ago. No leaks so far :) The rack is far more studier now. I have 48" bars on them and can carry 3 boats using the stacker, or two boats and a bike. I need wind fairing or the wind noise going under the front bar is horrible. Mileage absolutely sucks when loaded. On highways, I get 36 to 37 mpg (calculated, 40 to 41 indicated) with bare roof, 31 to 32 with just the rack, and 26 to 27 with three boats on top. It seriously lack power when loaded with three boats and interior filled with paddling and camping stuff. But overall it has been a great little car. It was never my primary paddling car so I don't know how it'll fare when used every weekend for paddling.
I would not put a raft on top, ever.
And if you ever do a western paddling trips with the car, the wind out west will seriously push the car around with boats on top.
Sometimes I carry my Speeder on the Fit, and it looks hilarious.