Yeah, I know a person who has her paint mare in a curb, and too much pressure (which wasn't much) on the bit she tossed her head really crazy. I suggested a snaffle, but the owner girl said snaffles were too harsh. Here's a plus: she didn't know what a belgian was! One horse I rode in a curb, he was fine, but didn't respond well to a snaffle, aka not enough pressure.
If D likes the bit, go with it before shaking things up anymore.