My long stock coat (out of short stock coated parents) has tons of drive and is not a mellow dog. He is a sweetie and loves to cuddle, but he definitely needs his exercise. He is very similar in personality to all of his siblings and half siblings that I have met and most of those are short coats.
I don't think length of coat has anything to do with it. I once read that all black shepherds were more mellow than other colors. I don't think that is true either