Are you one of 16 (somewhat obscure reference - my attempt at humor

).
From my experience:
1. Generally speaking, if you play outside or semi-outside (tent with open sides) you can never have too much power because the sound dissipates so much more than it does inside. I'm talking about amps, PA system, everything.
2. I've never played a multi-day festival where I've had to leave my gear overnight, but my wife and I have sold jewelry as vendors where we had to leave merchandise set up overnight. Generally speaking, security at night is good. We, nor any other vendor I can recall, have ever been ripped off that way. Weather is a bigger worry - if it looks bad I'd pack up at night if the sponsors will let you.
I hope I've answered the questions you asked.