If your mother is already shunning you then what is the point of getting df'd and letting them off the hook?
After the announcement people will spread gossip and slander about your immoral life. They will tell each other you left, not because there is something wrong with their religion, but because there was something wrong with you. If you DA and let people know why you are going to DA you can really stick it to them and maintain more dignity. Some people go that route. It may or may not be right for you.