Can't be bothered to read most of this thread, but sumo is a fantastic addition to anyone's training repetoire. So is judo.
Imagine being able to knock down your opponents with not just the power of your karate, but the knowledge of where and how to hit so your opponent falls off balance to the ground. Or being able to set up huge throws or pins with hard punches or kicks and knowledge of balance. You could take out people a lot bigger than you with this kind of knowledge.