I have a new to me 03 GT. I am an avid paddler and transport my boats quite often. Is it OK to transport 10' kayaks at around 40lbs using a j-bar rack on the top mounted luggage rack?
I think you will be fine with that sized boat on your car.
I regularly transport my 17 foot canoe on top of my 04 AWD vibe (it's on there right now actually). This includes 3+ hour trips at 80MPH on the highway. I have a kind of weird hybrid Thule rack setup on my car though that is (in my mind) a bit stronger then the stock setup.
I'm assuming you are planning on putting your j-bars directly on the factory racks and cross bars? How many boats are you planning on carrying at once? One boat seems like it would be pretty reasonable to me. Two might be pushing it on the factory cross bars... but you would probably be fine. Putting heftier cross bars on would make it no problem at all.
Post a picture of your setup once you've got it all loaded up!
2004 AWD Vibe -- slowly being built into a soft adventure vehicle.