I regularly do 10gl batches in a 15.5gl keggle. You will definitely need to attend to the kettle while bringing it up to a boil to prevent boil-overs, but it's certainly possible. Also, foam-control agents (Fermcap-S, simethicone/baby gas drops) will help, but are not fool-proof prevention.
One thing to note: you need a lot fewer BTUs to maintain a boil (and denature the proteins that will cause boil-overs), so you can get pretty far by "just" dropping the flame down once you start to see the boil form and the first signs of boil-over. After 15 minutes, you can start to ratchet the BTUs back up to get a solid, roiling boil, but without the boilover danger.