Bottom line: if deadly force is authorized, your weapon displayed and pointed at the perp is sufficient warning. I'm not even sure that words to the effect of "Leave Now" make sense -- particularly if one is not good at the growling command voice under very scary and stressul situations.
I agree weapon out and point should be the only warning given. Seeing #1 perp should not have been there to begin with #2 a warning shot lowers your round count #3 A lot of distance can be made up in a very short time. A short while ago a woman who lives near me walked in to her home as a perp was inside. He decided to shoot her on the way out.