I started Karate about a month back, I've been eating <20g carbs a day since october. I've never felt that I was too low on energy.
But what you're talking about doing is the same as TKD, where you carb up before work outs. It works, but it takes me forever to get back in ketosis.
Something else thats good for energy is ECA stacks, they always perk me up :P