Texas resident and never hear this.
"Y'all" means "you all", yes, but both small plural (2 people) or large plural (2,000,000,000 people). I've never heard a musician go on stage and say "How are all y'all doing tonight?". They just say "How are y'all doing tonight?"
"All y'all" really means "each of you". For example, if I'm running a bar and a family of parents and late teenagers enter, I would ask "are all y'all 18 or older?" Same if there are like 50 folks.
I cannot imagine a situation where I would say "all y'all" without it meaning to emphasize each and every one.

