I've never had problems with standard roof bars for carrying a 16 foot Old Town Canoe.
I also have a set of J-Bars which I can put on and carry a Pyrannah Mountain 300 Kayak at one end of the bars with the canoe.
All this on a small hatchback car. It gets some amazed looks but really hasn't been a problem to drive with or particularly noticeable on fuel consumption.
The only dodgy moments I have had are when it is very windy and there is a tendency to start 'sailing'
Just make sure the bars are wide enough. I got a new car last year and the garage supplied manufacturer's bars FOC. Unfortunately they are a lot shorter than my old ones and can only carry one boat. If I put the canoe on there is not a lot to spare!
If you are using roof bars don't forget to secure both ends to the towing points on your vehicle. I had a friend who didn't and the load came adrift on a UK motorway! As he looked out of his window he saw 4 kayaks peel off and bounce down the hard shoulder!! Amazingly no one was hurt and the boats only had minor scratches!
Carol